Old Tenant Supposedly Moving Issue

[Brandon Blankman]

So current situation: Old tenant is on a month to month lease. They had expressed they were purchasing a home back in October. Their lease rolled over into a month to month at the end of November. Old tenant had given me notice they planned to be out on Dec 15th. So naturally I started my process of finding a new tenant. As luck had it I found one with in hours of posting my listing. They were eager to move in as soon as they old tenants got things out starting on the 17th. So in order to help old tenants I just charged them a prorated amount through Dec 15. Just as things were going so smoothly they notified me yesterday that the contract on the prospective home might fall through due to some complications. Now they are saying they might be staying. So far I have been relaying information to the new tenants but if the old tenants decide to stay I would be losing out on a rent increase and risk the old tenants moving out in the near future and losing my very qualified new tenants. Any suggestions on how to proceed? 

[Brandon Blankman]

Updated: 

Was able to get the old tenants out by giving back their deposit so they can close early on the house. Took a small hit as a bedroom door was damaged but in the end I think it was worth the $100 hit to get them out and get the new tenants in. In all it was a quick turn around and things worked out. Also ended up with a nice charcoal grill that was left behind so there is a plus.