-7.500$ is shown because of the unlimited money plugin witch starts storing values at 8000$ and over, if you suddenly drop under 8000 the value is no longer stored and the game shows you a
"-" value to compensate for the loss in money. I have tested the plugin and it works fine both buy and earn $ work properly on my server.
However I had a another look at the plugin and I found out a crack in it, the plugin didn't cover a situation and so the error might have been generated ... apparently jetpacks and money don't fit very well 2 gather O.o so I had to user some super glue
The fix has been attached