First check your approval letter.

Normally, bank does not allow you to rent the house to the short sale seller over certain period (for instance, over 60 days is not permmitted). IF bank found out that you have rented it to the previous owner and IF bank wants to make your life tough, bank could. 

But, why do you want to rent it to the previous owner?  Do not do it if you are not closely related to them.