Don't overpay fee if we have selected new coins that result in a smaller transaction.

On repeated calls to SelectCoins we try to meet the fee necessary for the last transaction, the new fee required might be smaller, so increase our change by the difference if we can.

src/wallet/wallet.cpp

@@ -2534,8 +2534,25 @@ bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend, CWalletTx& wt
                     return false;
                 }
 
-                if (nFeeRet >= nFeeNeeded)
+                if (nFeeRet >= nFeeNeeded) {
+                    // Reduce fee to only the needed amount if we have change
+                    // output to increase.  This prevents potential overpayment
+                    // in fees if the coins selected to meet nFeeNeeded result
+                    // in a transaction that requires less fee than the prior
+                    // iteration.
+                    // TODO: The case where nSubtractFeeFromAmount > 0 remains
+                    // to be addressed because it requires returning the fee to
+                    // the payees and not the change output.
+                    // TODO: The case where there is no change output remains
+                    // to be addressed so we avoid creating too small an output.
+                    if (nFeeRet > nFeeNeeded && nChangePosInOut != -1 && nSubtractFeeFromAmount == 0) {
+                        CAmount extraFeePaid = nFeeRet - nFeeNeeded;
+                        vector<CTxOut>::iterator change_position = txNew.vout.begin()+nChangePosInOut;
+                        change_position->nValue += extraFeePaid;
+                        nFeeRet -= extraFeePaid;
+                    }
                     break; // Done, enough fee included.
+                }
 
                 // Try to reduce change to include necessary fee
                 if (nChangePosInOut != -1 && nSubtractFeeFromAmount == 0) {