Let's assume your statement prints on the first of every month. If U buy a $5000 air ticket in March, the CC will send your miles/points over on April 1. If U then refund it in April & also spend $5000 on your card in April, you will get 0 miles on May 1. If U spend <$5K, I assume U would get negative miles on May 1. If that withdrawal of miles puts your FF acct. below zero, U may be in trouble. It depends on whether the airline or hotel co. trusts U to earn to bring your balance up to zero.

Moral: I would consider doing it as a way of borrowing/ being advanced miles which I need earlier than I would really earn 'em, but not as a way of scamming for miles permanently.