Be careful with this. The owner is short selling because they don't have money to pay their mortgage. It's imperative that you have a conversation with all parties NOW about the owner's ability to pay you (and they should pay you something) to live in the property after Close of Escrow.
Pick a $ for 30 days of their continuing to live there & see if they would be willing to place those funds into your escrow account NOW. Then add $500-1000 to that # that you pick. Tell the owner, you will be holding this extra $500-1000 until they move out after 30 days. You would have the owners put money into an escrow account ONLY AFTER you've received an Approval on the Short Sale in writing.
They're not paying their mortgage, but hopefully they're still working & have an income. All this money they've been making & not paying their mortg. makes it easier for them to pay you some type of rent.
This extra money can be considered a "security deposit", which in their situation they'll surely want it back & that's the fire that will be lit under their butts to get OUT, so they get that portion of the money returned to them.