Lane Kiffin -- $500k Housing Mystery ... After USC Firing

USC didn't just take Lane Kiffin's dignity, the university might also be in a position to take his house ... TMZ has learned.

We broke the story ... after Lane accepted the head coaching job at the University of Southern California in 2010, the school agreed to loan him $500k to use toward the purchase of a $5.5 million mansion in Manhattan Beach, CA.

But there was a catch ... under the terms of the deal, if Lane got fired or quit during the life of the loan, USC could demand the $500k back, plus interest STAT ... and if he didn't pay up, SC could foreclose.

SC didn't disclose the length of the $500k note in official records ... and it's unclear if it's been forgiven.

However, these types of deals are usually made by an employer to keep an employee from bolting for greener pastures (like Lane's been known to do) ... and it's common for the term of the loan to be AT LEAST as long as the employment contract.

So, the question ... did Kiffin outlive the term of his $500k loan from SC, or is he still on the hook for the money?