I've had this same question. And what follows is in NO WAY legal advice or expert advice or even good advice... just some thoughts.
First, it makes perfect sense to me that the buyers move in. They'll take care of the home, pay the utilities, make sure it doesn't get vandalized and of course, become more committed to the home. Of course there are downsides anytime you do an early possession, but in the case of a short sale that's vacant, I think the positives outweigh the risks.
However, collecting rent is questionable. The homeowner isn't paying the mortgage, in all likelihood and in some circles, this practice is considered illegal. It's not likely that he'd logistically be able to give the money to the bank.
If it were my property, I'd probably just let the renters move in and take over all bills and maintenance on the property.
Again, this is not legal or expert advice!
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Jennifer Allan, GRI
RE/MAX Hall of Fame
Author of
Sell with Soul, Creating an Extraordinary Career in Real Estate without Losing Your Friends, Your Principles or Your Self-Respect