From what I can gather it seems to be mutually parasitic:
1. Jimmy is better than free labour for Hoarderman, he doesn't have to pay him, and Jimmy actually pays off Hoarderman's expenses
2. Jimmy believes he's going to get a major windfall out of Hoarderman. I can only assume that Hoarderman has no next of kin or family to leave money to, so Jimmy probably assumes that all of the money that he earns for Hoarderman, and presumably the property, will end up with him once Hoarderman dies and is just biding his time
The issue with the second point, however, is that Hoarderman spends all his money on weed by the sounds of it, so realistically Jimmy may only get the land (if Hoarderman has nobody else to leave it to). It may well be that Jimmy is just selling off the least valuable stuff and retaining the more valuable items so that he can sell it once Hoarderman dies.