pay a dollar
take a piss (especially if it is an inconvenience to do so)
Dude, get off at the next exit and find a gas station. I gotta pay a dollar.
paying negative dollars
buying clothes (normally used) for a certain price, then finding more money in the pocket than what you originally paid for them
I bought a $20 pair of jeans at Plato's Closet the other day and found a wadded up $50 dollar bill in one of the pockets when I got home. I love paying negative dollars for clothes.