We don't know for sure -- it's not verified that he went to the atm, but very little seems to be verified -- I've read that he went there 2 times that night.
No idea what the money would be for, though at that point, the "boys" weren't being looked at by officials, nobody was. She wasn't even officially missing at that point.
Actually, at that time P's good friend was still the polis chief, so J wasn't in too much trouble. The chief couldn't bring himself later to lift a finger, until he absolutely had to.
Paying of someone to help dispose of Natalee's body perhaps?