diff --git a/src/Modules/Billing/Modules.Billing/Domain/Wallet.cs b/src/Modules/Billing/Modules.Billing/Domain/Wallet.cs
index 4aac34a6d8..034b7fc04e 100644
--- a/src/Modules/Billing/Modules.Billing/Domain/Wallet.cs
+++ b/src/Modules/Billing/Modules.Billing/Domain/Wallet.cs
@@ -31,26 +31,27 @@ public static Wallet Create(string tenantId, string currency)
         };
     }
 
-    public WalletTransaction Credit(decimal amount, WalletTransactionKind kind, string description, string? referenceId)
+    public WalletTransaction Credit(Money amount, WalletTransactionKind kind, string description, string? referenceId)
     {
-        ArgumentOutOfRangeException.ThrowIfLessThanOrEqual(amount, 0m);
-        var credit = new Money(amount, Balance.Currency);
-        var tx = WalletTransaction.Create(Id, TenantId, credit, kind, description, referenceId);
+        ArgumentNullException.ThrowIfNull(amount);
+        ArgumentOutOfRangeException.ThrowIfLessThanOrEqual(amount.Amount, 0m);
+        var tx = WalletTransaction.Create(Id, TenantId, amount, kind, description, referenceId);
         _transactions.Add(tx);
-        Balance = Balance.Add(credit);
+        Balance = Balance.Add(amount);
         UpdatedAtUtc = DateTime.UtcNow;
         return tx;
     }
 
-    public WalletTransaction Debit(decimal amount, WalletTransactionKind kind, string description, string? referenceId)
+    public WalletTransaction Debit(Money amount, WalletTransactionKind kind, string description, string? referenceId)
     {
-        ArgumentOutOfRangeException.ThrowIfLessThanOrEqual(amount, 0m);
-        if (amount > Balance.Amount)
+        ArgumentNullException.ThrowIfNull(amount);
+        ArgumentOutOfRangeException.ThrowIfLessThanOrEqual(amount.Amount, 0m);
+        var remaining = Balance.Subtract(amount);
+        if (remaining.Amount < 0m)
             throw new InvalidOperationException("Insufficient wallet balance.");
-        var debit = new Money(amount, Balance.Currency);
-        var tx = WalletTransaction.Create(Id, TenantId, new Money(-amount, Balance.Currency), kind, description, referenceId);
+        var tx = WalletTransaction.Create(Id, TenantId, amount with { Amount = -amount.Amount }, kind, description, referenceId);
         _transactions.Add(tx);
-        Balance = Balance.Subtract(debit);
+        Balance = remaining;
         UpdatedAtUtc = DateTime.UtcNow;
         return tx;
     }
diff --git a/src/Modules/Billing/Modules.Billing/Services/BillingService.cs b/src/Modules/Billing/Modules.Billing/Services/BillingService.cs
index 0a6643d140..7b8aae2eda 100644
--- a/src/Modules/Billing/Modules.Billing/Services/BillingService.cs
+++ b/src/Modules/Billing/Modules.Billing/Services/BillingService.cs
@@ -1,3 +1,4 @@
+using System.Net;
 using Finbuckle.MultiTenant.Abstractions;
 using FSH.Framework.Core.Exceptions;
 using FSH.Framework.Eventing.Abstractions;
@@ -255,9 +256,16 @@ public async Task MarkInvoicePaidAsync(Guid invoiceId, CancellationToken cancell
                     wallet = Wallet.Create(invoice.TenantId, invoice.Currency);
                     _db.Wallets.Add(wallet);
                 }
+                else if (!string.Equals(wallet.Currency, invoice.Currency, StringComparison.Ordinal))
+                {
+                    throw new CustomException(
+                        $"Cannot credit a {invoice.Currency} top-up to a {wallet.Currency} wallet.",
+                        (IEnumerable<string>?)null,
+                        HttpStatusCode.Conflict);
+                }
 
                 wallet.Credit(
-                    invoice.SubtotalAmount.Amount,
+                    invoice.SubtotalAmount,
                     WalletTransactionKind.Topup,
                     "WhatsApp wallet top-up",
                     topupRequest.Id.ToString());
diff --git a/src/Tests/Billing.Tests/Domain/WalletTests.cs b/src/Tests/Billing.Tests/Domain/WalletTests.cs
index 36ef5e9ae6..5b9fa7cf25 100644
--- a/src/Tests/Billing.Tests/Domain/WalletTests.cs
+++ b/src/Tests/Billing.Tests/Domain/WalletTests.cs
@@ -1,3 +1,4 @@
+using FSH.Framework.Core.Domain;
 using FSH.Modules.Billing.Contracts;
 using FSH.Modules.Billing.Domain;
 using Shouldly;
@@ -22,7 +23,7 @@ public void Create_starts_active_with_zero_balance()
     public void Credit_increases_balance_and_returns_ledger_row()
     {
         var w = Wallet.Create("tenant-a", "USD");
-        var tx = w.Credit(50m, WalletTransactionKind.Topup, "Top-up", "req-1");
+        var tx = w.Credit(new Money(50m, "USD"), WalletTransactionKind.Topup, "Top-up", "req-1");
         w.Balance.Amount.ShouldBe(50m);
         tx.Amount.Amount.ShouldBe(50m);
         tx.WalletId.ShouldBe(w.Id);
@@ -33,14 +34,22 @@ public void Credit_increases_balance_and_returns_ledger_row()
     [Fact]
     public void Credit_rejects_non_positive_amount()
         => Should.Throw<ArgumentOutOfRangeException>(
-            () => Wallet.Create("t", "USD").Credit(0m, WalletTransactionKind.Topup, "x", null));
+            () => Wallet.Create("t", "USD").Credit(new Money(0m, "USD"), WalletTransactionKind.Topup, "x", null));
+
+    [Fact]
+    public void Credit_rejects_currency_mismatch()
+    {
+        var w = Wallet.Create("tenant-a", "USD");
+        Should.Throw<InvalidOperationException>(
+            () => w.Credit(new Money(50m, "EUR"), WalletTransactionKind.Topup, "Top-up", null));
+    }
 
     [Fact]
     public void Debit_beyond_balance_throws()
     {
         var w = Wallet.Create("tenant-a", "USD");
-        w.Credit(10m, WalletTransactionKind.Topup, "Top-up", null);
+        w.Credit(new Money(10m, "USD"), WalletTransactionKind.Topup, "Top-up", null);
         Should.Throw<InvalidOperationException>(
-            () => w.Debit(25m, WalletTransactionKind.MessageCharge, "msg", null));
+            () => w.Debit(new Money(25m, "USD"), WalletTransactionKind.MessageCharge, "msg", null));
     }
 }
diff --git a/src/Tests/Billing.Tests/Mappings/WalletMappingTests.cs b/src/Tests/Billing.Tests/Mappings/WalletMappingTests.cs
index ad613f2ef1..a5272b8e89 100644
--- a/src/Tests/Billing.Tests/Mappings/WalletMappingTests.cs
+++ b/src/Tests/Billing.Tests/Mappings/WalletMappingTests.cs
@@ -1,3 +1,4 @@
+using FSH.Framework.Core.Domain;
 using FSH.Modules.Billing.Domain;
 using FSH.Modules.Billing.Mappings;
 using FSH.Modules.Billing.Contracts;
@@ -12,7 +13,7 @@ public sealed class WalletMappingTests
     public void Wallet_ToDto_emits_string_status_and_balance()
     {
         var w = Wallet.Create("tenant-a", "USD");
-        w.Credit(50m, WalletTransactionKind.Topup, "Top-up", "req-1");
+        w.Credit(new Money(50m, "USD"), WalletTransactionKind.Topup, "Top-up", "req-1");
         var dto = w.ToDto();
         dto.Balance.ShouldBe(50m);
         dto.Status.ShouldBe("Active");