?

I was teaching my friend how to trade in game, he is a new account and only lvl5, i traded him 20M gold in order to let him understand the whole process of a trade in this game, after that, he wants to trade the 20M gold back to me, but the system said he can only give away 2,5000 coin, why is that? Of course i could just give the 20m to my friend, but i just want to figure this out why he can only give away 2,5000 coins...?