Here's the thing
A) You state, you made a verbal agreement. Are you sure your "Friend" Comprehended this verbal statement?
5K is alot of money, and ppl do tend to be, more defensve towards the money in a sense, there-fore, I really think if it were only like 1k a piece he would'nt be so up-tight about this. None-the-less, a deal is still a deal.
With that being said and done, I think, you should take the money, and tell your friend, if you want some money back then lets have a HU, cash game, 2-4blinds, 1 on 1, and 1k buyin, there-fore, if he loses that 1k, then you can just make him buy-in again, and again, untill the whole 10k is yours, then tell him...................................
Well friend, how does it feel !
But then again that's just me