When a pass of CreateTransaction fails to select enough bitcoin to meet the nFeeNeeded the next pass through may select a far smaller collection of inputs.  Currently it will continue to use the old fee amount based on the larger set of inputs, which can cause a non-trivial over-payment.

This may be the cause of the behavior reported here: https://www.reddit.com/r/Bitcoin/comments/5ltw5n/bitcoin_core_v0131_sends_enormously_high_fee/

Long ago fixing this was tricky due to a catch-22 between signing the transaction and knowing how much fee was required.  But after #9465 the whole algorithm will run using a dummy signature, so when there is already a change output fixing up the fee after finding a sufficient transaction should be a matter of a ~2 line change.

if (recipient.fSubtractFeeFromAmount && nFeeRet > 0)
                        {
                            if (txout.nValue < 0)
                                strFailReason = _("The transaction amount is too small to pay the fee");
                            else
                                strFailReason = _("The transaction amount is too small to send after the fee has been deducted");
                        }
                        else
                            strFailReason = _("Transaction amount too small");
                        return false;
                    }
                    txNew.vout.push_back(txout);
                }
                
                CAmount nValueIn = 0;
                setCoins.clear();
                if (!SelectCoins(vAvailableCoins, nValueToSelect, setCoins, nValueIn, coinControl))
                {
                    strFailReason = _("Insufficient funds");
                    return false;
                }
                for (const auto& pcoin : setCoins)
                {
                    CAmount nCredit = pcoin.first->tx->vout[pcoin.second].nValue;
                    
                    
                    
                    
                    int age = pcoin.first->GetDepthInMainChain();
                    assert(age >= 0);