Sale pending, how to construct rental lease?

[Shaoqing Xu]

Please help on how to construct a right lease for new tenant in below special situation:

I have a potential buyer of the condo, pending on his daughter admission into a local university. He won’t know the addmission result until end of Dec. If he don’t get admission, he will not purchase the condo unit. I’ll definitely go ahead for the sale if all goes smoothly.

In same time, I’m prepare to rent out the unit in case the sale fall apart, and have a good tenant ready to sign a lease now for Jan 20th move in. How to maybe put some right clause in lease, or right explanation to tenant so I can back out later Dec if needed without causing much trouble for tentant and I? 

Thanks.