fix: correct fee calculations in CreateTransactionInternal

Adapted from 87e0ef9 in bitcoin#25647 and c1a84f1 in bitcoin#26643

Co-authored-by: UdjinM6 <UdjinM6@users.noreply.github.com>

Author: kwvg

src/wallet/spend.cpp

@@ -625,6 +625,7 @@ bool CWallet::CreateTransactionInternal(
     CMutableTransaction txNew;
     FeeCalculation feeCalc;
     int nBytes{0};
+    CAmount fee_needed{0};
     {
         std::set<CInputCoin> setCoins;
         LOCK(cs_wallet);
@@ -806,9 +807,8 @@ bool CWallet::CreateTransactionInternal(
                 nBytes += GetSizeOfCompactSize(nExtraPayloadSize) + nExtraPayloadSize;
             }
 
-            nFeeRet = coin_selection_params.m_effective_feerate.GetFee(nBytes);
+            fee_needed = coin_selection_params.m_effective_feerate.GetFee(nBytes);
 
-            CAmount fee_needed = nFeeRet;
             if (nSubtractFeeFromAmount == 0) {
                 change_position->nValue -= fee_needed;
             }
@@ -828,6 +828,8 @@ bool CWallet::CreateTransactionInternal(
                 fee_needed = coin_selection_params.m_effective_feerate.GetFee(nBytes);
             }
 
+            nFeeRet = inputs_sum - nValue - change_amount;
+
             // Update nFeeRet in case fee_needed changed due to dropping the change output
             if (fee_needed <= change_and_fee - change_amount) {
                 nFeeRet = change_and_fee - change_amount;
@@ -903,6 +905,11 @@ bool CWallet::CreateTransactionInternal(
         }
     }
 
+    if (fee_needed > nFeeRet) {
+        error = _("Fee needed > fee paid");
+        return false;
+    }
+
     if (nFeeRet > m_default_max_tx_fee) {
         error = TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED);
         return false;