diff --git a/apps/api/src/api/services/monerium/index.ts b/apps/api/src/api/services/monerium/index.ts
index a01e25965..e237e5cb0 100644
--- a/apps/api/src/api/services/monerium/index.ts
+++ b/apps/api/src/api/services/monerium/index.ts
@@ -68,7 +68,7 @@ export const checkAddressExists = async (address: string, network: Networks): Pr
     return null;
   } catch (error) {
     logger.error("Failed to fetch address:", error);
-    return null;
+    throw error;
   }
 };
 
@@ -168,12 +168,11 @@ export const getMoneriumUserIban = async ({ authToken, profileId }: FetchIbansPa
       throw new Error(`API request failed with status ${response.status}: ${response.statusText}`);
     }
     const data: IbanDataResponse = await response.json();
-    // Look for the IBAN data specifically for the Polygon chain.
-    // We choose Polygon as the default chain for Monerium EUR minting,
-    // so user registered with us should always have a Polygon-linked  address.
-    const ibanData = data.ibans.find(item => item.chain === "polygon");
+    // Look for the IBAN data specifically for the configured Monerium mint chain.
+    // This aligns with sandbox/prod chain selection.
+    const ibanData = data.ibans.find(item => item.chain === MONERIUM_MINT_CHAIN);
     if (!ibanData) {
-      throw new Error("No IBAN found for the specified chain (polygon)");
+      throw new Error(`No IBAN found for the specified chain (${MONERIUM_MINT_CHAIN})`);
     }
 
     return ibanData;
diff --git a/apps/api/src/api/services/quote/core/quote-fees.ts b/apps/api/src/api/services/quote/core/quote-fees.ts
index ef413bdf7..9196d707b 100644
--- a/apps/api/src/api/services/quote/core/quote-fees.ts
+++ b/apps/api/src/api/services/quote/core/quote-fees.ts
@@ -78,6 +78,7 @@ async function calculatePartnerAndVortexFees(
 ): Promise<{ partnerMarkupFee: Big; vortexFee: Big }> {
   let totalPartnerMarkupInFeeCurrency = new Big(0);
   let totalVortexFeeInFeeCurrency = new Big(0);
+  let shouldApplyDefaultVortexFees = !partnerId;
 
   // 1. Fetch and process partner-specific configurations if partnerName is provided
   if (partnerId) {
@@ -142,15 +143,17 @@ async function calculatePartnerAndVortexFees(
       // Log warning if partner found but no applicable custom fees
       if (!hasApplicableFees) {
         logger.warn(`Partner with name '${partnerId}' found, but no active markup defined. Proceeding with default fees.`);
+        shouldApplyDefaultVortexFees = true;
       }
     } else {
       // No specific partner records found, will use default Vortex fee below
       logger.warn(`No fee configuration found for partner with name '${partnerId}'. Proceeding with default fees.`);
+      shouldApplyDefaultVortexFees = true;
     }
   }
 
   // 2. If no partner was provided initially, use default Vortex fees
-  if (!partnerId) {
+  if (shouldApplyDefaultVortexFees) {
     // Query all vortex records for this ramp type
     const vortexFoundationPartners = await Partner.findAll({
       where: {
diff --git a/apps/api/src/api/services/quote/engines/fee/index.ts b/apps/api/src/api/services/quote/engines/fee/index.ts
index 37ba1c250..772b8e27a 100644
--- a/apps/api/src/api/services/quote/engines/fee/index.ts
+++ b/apps/api/src/api/services/quote/engines/fee/index.ts
@@ -43,6 +43,13 @@ export abstract class BaseFeeEngine implements Stage {
 
     this.validate(ctx);
 
+    if (request.rampType === RampDirection.SELL && !ctx.nablaSwap) {
+      throw new APIError({
+        message: "Missing nabla swap output amount for off-ramp fee calculation",
+        status: httpStatus.BAD_REQUEST
+      });
+    }
+
     const { anchorFee, feeCurrency, partnerMarkupFee, vortexFee } = await calculateFeeComponents({
       from: request.from,
       inputAmount: request.inputAmount,
diff --git a/apps/api/src/api/services/ramp/ramp.service.ts b/apps/api/src/api/services/ramp/ramp.service.ts
index 08bd1939e..172c2aad4 100644
--- a/apps/api/src/api/services/ramp/ramp.service.ts
+++ b/apps/api/src/api/services/ramp/ramp.service.ts
@@ -38,7 +38,7 @@ import RampState from "../../../models/rampState.model";
 import TaxId from "../../../models/taxId.model";
 import { APIError } from "../../errors/api-error";
 import { ActivePartner, handleQuoteConsumptionForDiscountState } from "../../services/quote/engines/discount/helpers";
-import { createEpcQrCodeData, getIbanForAddress, getMoneriumUserProfile } from "../monerium";
+import { createEpcQrCodeData, getIbanForAddress, getMoneriumUserProfile, MONERIUM_MINT_CHAIN } from "../monerium";
 import { StateMetadata } from "../phases/meta-state-types";
 import phaseProcessor from "../phases/phase-processor";
 import { prepareOfframpTransactions } from "../transactions/offramp";
@@ -238,7 +238,8 @@ export class RampService extends BaseRampService {
 
       presignedTxs.forEach((newTx: UnsignedTx) => {
         const existingIndex = updatedTxs.findIndex(
-          tx => tx.phase === newTx.phase && tx.network === newTx.network && tx.signer === newTx.signer
+          tx =>
+            tx.phase === newTx.phase && tx.network === newTx.network && tx.signer === newTx.signer && tx.nonce === newTx.nonce
         );
         if (existingIndex >= 0) {
           updatedTxs[existingIndex] = newTx;
@@ -888,7 +889,7 @@ export class RampService extends BaseRampService {
       const ibanData = await getIbanForAddress(
         additionalData.moneriumWalletAddress,
         additionalData.moneriumAuthToken,
-        quote.to as EvmNetworks // Fixme: assethub network type issue.
+        MONERIUM_MINT_CHAIN
       );
 
       const userProfile = SANDBOX_ENABLED
diff --git a/apps/api/src/api/services/transactions/stellar/offrampTransaction.ts b/apps/api/src/api/services/transactions/stellar/offrampTransaction.ts
index eb6f4dea8..939954058 100644
--- a/apps/api/src/api/services/transactions/stellar/offrampTransaction.ts
+++ b/apps/api/src/api/services/transactions/stellar/offrampTransaction.ts
@@ -114,7 +114,7 @@ export async function buildPaymentAndMergeTx({
     currentCreateAccountTransaction.sign(fundingAccountKeypair);
 
     createAccountTransactions.push({
-      sequence: fundingAccount.sequenceNumber(), // TODO do we require this?
+      sequence: currentFundingAccount.sequenceNumber(), // TODO do we require this?
       tx: currentCreateAccountTransaction.toEnvelope().toXDR().toString("base64")
     });
   }
diff --git a/apps/api/src/api/services/transactions/validation.ts b/apps/api/src/api/services/transactions/validation.ts
index f2ef3fd54..c93f5baf5 100644
--- a/apps/api/src/api/services/transactions/validation.ts
+++ b/apps/api/src/api/services/transactions/validation.ts
@@ -21,6 +21,56 @@ import QuoteTicket from "../../../models/quoteTicket.model";
 import { APIError } from "../../errors/api-error";
 
 /// Checks if all the transactions in 'subset' are contained in 'set' based on phase, network, nonce, and signer.
+function isEvmTransactionLike(
+  data: unknown
+): data is {
+  to: string;
+  data: string;
+  value: string;
+  gas: string;
+  maxFeePerGas?: string;
+  maxPriorityFeePerGas?: string;
+  nonce?: number;
+} {
+  return typeof data === "object" && data !== null && "to" in data && "data" in data;
+}
+
+function txDataMatches(setTxData: PresignedTx["txData"], subsetTxData: PresignedTx["txData"]): boolean {
+  if (setTxData === subsetTxData) {
+    return true;
+  }
+
+  if (typeof setTxData === "string" && typeof subsetTxData === "string") {
+    return setTxData === subsetTxData;
+  }
+
+  if (isEvmTransactionLike(setTxData) && isEvmTransactionLike(subsetTxData)) {
+    return JSON.stringify(setTxData) === JSON.stringify(subsetTxData);
+  }
+
+  const signed = typeof setTxData === "string" ? setTxData : typeof subsetTxData === "string" ? subsetTxData : null;
+  const unsigned = isEvmTransactionLike(setTxData) ? setTxData : isEvmTransactionLike(subsetTxData) ? subsetTxData : null;
+
+  if (signed && unsigned) {
+    try {
+      const parsed = EvmTransaction.from(signed);
+      return (
+        parsed.to?.toLowerCase() === unsigned.to.toLowerCase() &&
+        parsed.data === unsigned.data &&
+        parsed.value?.toString() === unsigned.value &&
+        parsed.gasLimit?.toString() === unsigned.gas &&
+        parsed.maxFeePerGas?.toString() === unsigned.maxFeePerGas &&
+        parsed.maxPriorityFeePerGas?.toString() === unsigned.maxPriorityFeePerGas &&
+        parsed.nonce === unsigned.nonce
+      );
+    } catch {
+      return false;
+    }
+  }
+
+  return false;
+}
+
 export function areAllTxsIncluded(subset: PresignedTx[], set: PresignedTx[]): boolean {
   for (const subsetTx of subset) {
     const match = set.find(
@@ -28,7 +78,8 @@ export function areAllTxsIncluded(subset: PresignedTx[], set: PresignedTx[]): bo
         setTx.phase === subsetTx.phase &&
         setTx.network === subsetTx.network &&
         setTx.nonce === subsetTx.nonce &&
-        setTx.signer === subsetTx.signer
+        setTx.signer === subsetTx.signer &&
+        txDataMatches(setTx.txData, subsetTx.txData)
     );
 
     if (!match) {
@@ -122,7 +173,15 @@ function validateEvmTransaction(tx: PresignedTx, expectedSigner: string) {
     });
   }
 
-  const transactionMeta = EvmTransaction.from(txData);
+  let transactionMeta: EvmTransaction;
+  try {
+    transactionMeta = EvmTransaction.from(txData);
+  } catch (error) {
+    throw new APIError({
+      message: `Invalid EVM transaction data: ${(error as Error).message}`,
+      status: httpStatus.BAD_REQUEST
+    });
+  }
   if (!transactionMeta.from) {
     throw new APIError({
       message: "EVM transaction data must be signed and include a 'from' address",
@@ -157,6 +216,12 @@ async function validateSubstrateTransaction(tx: PresignedTx, expectedSignerSubst
 
   if (tx.phase === "moonbeamToPendulumXcm" || tx.phase === "moonbeamCleanup") {
     // Moonbeam uses EVM addresses but the transactions are Substrate-based
+    if (!expectedSignerEvm) {
+      throw new APIError({
+        message: `Expected EVM signer for Substrate transaction is not provided for phase ${tx.phase}`,
+        status: httpStatus.BAD_REQUEST
+      });
+    }
     if (signer.toLowerCase() !== expectedSignerEvm.toLowerCase()) {
       throw new APIError({
         message: `Substrate transaction signer ${signer} does not match the expected signer ${expectedSignerEvm} for phase ${tx.phase}.`,
diff --git a/apps/frontend/src/machines/actors/sign.actor.ts b/apps/frontend/src/machines/actors/sign.actor.ts
index 769e751d0..1eb013587 100644
--- a/apps/frontend/src/machines/actors/sign.actor.ts
+++ b/apps/frontend/src/machines/actors/sign.actor.ts
@@ -95,7 +95,11 @@ export const signTransactionsActor = async ({
   // Monerium onramp
   if (rampDirection === RampDirection.SELL && quote?.from === "sepa") {
     if (!getMessageSignature) throw new Error("getMessageSignature not available");
-    const offrampMessage = await MoneriumService.createRampMessage(rampState.quote.outputAmount, "THIS WILL BE THE IBAN");
+    const iban = rampState.ramp?.ibanPaymentData?.iban;
+    if (!iban) {
+      throw new Error("Missing IBAN for Monerium offramp signature");
+    }
+    const offrampMessage = await MoneriumService.createRampMessage(rampState.quote.outputAmount, iban);
     moneriumOfframpSignature = await getMessageSignature(offrampMessage);
   }
 
diff --git a/packages/shared/src/endpoints/ramp.endpoints.ts b/packages/shared/src/endpoints/ramp.endpoints.ts
index 6ce16a4b1..29943dbe0 100644
--- a/packages/shared/src/endpoints/ramp.endpoints.ts
+++ b/packages/shared/src/endpoints/ramp.endpoints.ts
@@ -52,7 +52,8 @@ export interface AccountMeta {
 
 export interface EvmTransactionData {
   to: EvmAddress;
-  data: EvmAddress;
+  // Calldata hex string (0x...)
+  data: string;
   value: string;
   gas: string;
   maxFeePerGas?: string;