I'd try to give them a call and explain the coupon. Are you sure that you are pre-paying when you book online? More often than not, they may "authorize" your card or charge you for the first night of a multi-night stay, but usually the majority of your stay isn't actually charged until check-out.
If you are only staying for one night, and they charge for the first night, they can probably just refund you the $50 at check-out.
UPDATE - I did a quick reservation quote on Palms.com. Based on what I found, I am guessing you are staying for only one night? Palms always charges one-night to your card as a deposit. If you are staying multiple nights, the remainder won't be charged until checkout, so you wouldn't have any issues using the coupon at checkout.
If you are staying only one night, it is best if you call and get it direct from the source, but again, I suspect they would simply be able to refund 50% (up to $50) back to your card at checkout.
Brent