Sometimes it can take a little bit of time for the R$ transfer to occur. If it doesn't show up in the next day or so, recheck your trade history(In left bar -> Trade -> Completed -> View Details(on the trade) and make sure everything checks out.)
If everything checks out then go to Trade -> My Transactions -> Purchases and see what you have bought.
If you did not buy anything with your ROBUX, then send ROBLOX an email: [email protected] or ask around on Twitter!