Update: I was wrong. It's not possible to keep the quick cash gift cards going at the same time as PayAll payments > credit limit even if you very carefully time payments to the card to ensure it is never put into credit. Citibank appear to direct payments to paying off any outstanding FPO's if not first, then definitely not last either. This month as an experiment I took out another quick cash FPO at $22.50 cost, but this time when it came round to funding the scheduled PayAll payments, I was very, very careful not to put the account into actual credit at any point. Even despite this, Citibank have cancelled and paid out the outstanding FPO, hence wasting another $22.50.
Fortunately I have another card with a much higher limit that can withstand the draw on it in conjunction with multiple FPO's, so that one is still getting the free gift cards which will cover the loss of 3 x $22.50 on the other one, but lesson learnt. I will not attempt a fourth quick cash FPO claim on the other one, since Citibank's rules around allocating funds to early repayment and cancellation are too opaque and arbitrary to be trusted.