If you have a credit balance on your statement meaning you did more returns than purchases the CC company will usually send a negative amount to the FF program. I'm not positive about Starwoods but it certainly works that way with most Bank of America cards.
The only question as I see it is about bonus miles for hitting spending thresholds such as the $15k with Starwoods. If you return the goods, will the points be deducted? Even if they do, I believe you can transfer the points quickly from one Starwoods account to another. I can't imagine they can deduct points once already transferred. If this is the case, there are many ways to take advantage of these promos.