HELP!! Weird pre-lease situation

[Janeen Archer]

I had a lease signing scheduled today with a couple who are selling their home to build one and were going to lease from me for 18 months while buying land and building. They check out as the owners of the property but said a pending offer fell through and didn't want to sign the lease for 18 months in case something bad happened and they couldn't sell. (this is Seattle...they can't NOT sell unless they're doing something very wrong) but wanted to give me one months rent, $2200 to rent it for 1 month thinking it should be sold by then. They claim they will not move in so I don't have to re-clean if I need to re lease it which they've stated a date that I could start advertising it. Once they sell they will sign a lease for the remaining. How do I legally rent it for one month, then release it for the remaining period. Is there a danger of something horrible happening by doing something as minor as not sending them a renewal letter on a one month lease?