If you are renting a property, and before the lease is up you buy the property from the owner, when does the owner have to return to security deposit? (At closing or does he have 30 days)

This is a rather interesting situation. Most state laws specify a return perod based on either when the lease ends or the tenant vacates. The clock starts at that point and runs for a period (usually 30 days or less) depending on the area. I don't think the laws specifically address this situation. So it should be an issue of negotiation. You should have an attorney representing you at closing. I would make sure its stipulated that you will deduct the security from the amount being paid to the former owner. There should be no issue of unpaid rents or damage to the property, since any damages would become the responsibility of the new owner (the former tenant) and the security deposit would nt be required by the landlord since he would not be responsible for any expense.

At closing.

Yes, at closing, it is not actually "returned' but would be added in to the closing statement as a credit.