LO And Keep Home On MLS?

We have been trying to sell our existing home in FL since July. We have had a few offers but none that worked for us. We now have a couple interested in doing and Lease Option. We would rather sell the property.
My question is if someone pays the option fee, and we keep the house on the market, and say 2 months after they have moved in, we receive an offer. Our TB would have first right of refusal. If they are not able to get financing in place, do you keep the option fee? Also, what is a reasonable length of time to give the TB for vacating the property.