What happened with the diamond money after Jonny gave it to Jim?
I would assume that Jim and the other biker stashed it somewhere, but when Niko killed them both, the secret died with them.
I dont know, but what i do remember is that the actual diamonds got lost in the thrash bags, and in th ending scene of tbogt (SPOILER ALERT) after killing Bulgarin luis meets with Tony and Yusuf Amir in a park in Dukes. when he approaches the bench tony sits on he accidently knocks over a drifter who falls in a pile of thrash, which just happens to be the thrash containing the diamonds. the drifter/tramp spots them and runs of with them, without luis notices whats going on. i remember that later i read a report on the ingame website of the liberty tree newspaper that a tramp found diamonds and had become a millionaire afterselling them