Purchasing a property with a rent free tenant

Hi,

I am purchasing a property and there is one tenant in the building that is living there rent-free since they work for the owner , they told me that they either can have him vacate the property at closing or I can have him sign a lease and pay me the rent.

I was wondering how you would go about it how can I make sure he is going to pay after I purchase the property, is a possible for me to sign a lease while I still am only in contract and don't own the property , How would you go about in this situation?