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 (GetTime() < nNextResend || !fBroadcastTransactions)
        return;
    bool fFirst = (nNextResend == 0);
    nNextResend = GetTime() + GetRand(30 * 60);
    if (fFirst)
        return;
    
    if (nBestBlockTime < nLastResend)
        return;
    nLastResend = GetTime();
    
    
    std::vector<uint256> relayed = ResendWalletTransactionsBefore(nBestBlockTime-5*60, connman);
    if (!relayed.empty())
        LogPrintf("%s: rebroadcast %u unconfirmed transactions\n", __func__, relayed.size());
}
 
/** @defgroup Actions
 *
 * @{
 */
CAmount CWallet::GetBalance() const