Pauline, You had NOTHING in writing and the tenant gave you $11,500 so YOU could make up your back mortgage payments??!! Did you give him a promisorry note for the $11,500? Does he currently have a lease for the condo? When is it up? How would he KNOW you were going to make-up the back payments? The deal is as scattered as your posting's information. Do you intend on giving him BACK the $11,500? This is precisely why doing anything with a FSBO is just asking for trouble!!
I would try and talk with him but you don't seem to have a clue how to conduct a real estate transaction and I fear you're going to bury yourself. You may wish to have an attorney sort this whole thing out. You've left yourself open to all sorts of litigation.