If you choose to take gold instead of something from Rebxn's bag of tricks, you get 50 additional XP. Probably not intentional - or at the very least, it should be included in the actual quest completion XP so everyone gets it.

i think its more because you are smart enough not to trust a goblin's bag of tricks, >_> so they congratulate you for it.

Well, it's not a bug. The reason has more to do with convenience in terms of interacting with how the quest system works and not adding unnecessary scripts.