(Unsure if there's a template for ideas/suggestions but here it goes)
Uppon donating the player would get a small balance reward, e.g: Today I bought Lapis, would be nice if it came with like 50,000 in-game money!
It's a pretty simple concept, and doesn't hurt the economy at all. I think this would be an overall positive thing for the ones who are helping the server out. Essentially a small thank you, besides the other perks!