Or maybe the payment initially went through to pending status but then another purchase went through first and put them over thier credit limit or debit card balance.
I would've expected the steam transaction to be reversed and the game removed from your library rather than showing a negative balance though, but maybe it was for a microtransaction or something.
This seems like the cost of a game that they are unable to charge op for. Do you feel like they are not entitled to the money owed?
They could have just removed the game from the account, at least this way op is alerted of the issue and given a chance to correct it without an interruption to his entertainment.
You're only allowed to play the game "Debtors' Prison" until it's paid off. You get $0.18/hour of play but 20% interest is compounded daily on the remaining balance.