Can you call the Casino desk and book and apply the $500. I do this and never had a problem.
I had recently a different experience. Had booked an 11 niter Explorer on board and later transferred to my TA. At the end of my Oasis cruise, received a credit certificate of $200 from the Casino. The Monday after I got home, called my TA and asked them if I can apply the credit to the Explorer sail. My contact was hospitable and said that she needed to contact Royal for approval. A few days later got an e-mail telling me that the transaction was approved and processed. I had to give back $50 in TAs OBC. However, I was positive $150.