I got a reasonable discount on the purchase price, and it has made me a good amount of money over the years.

Divorce - its easier if the house is in the divorce decree otherwise you need to play mediator between parties to come to an agreement. 

If not, I'm guessing it would be treated as a traditional rental loan, either conventional or DSCR, using market rents rather than the rent in the lease agreement.

(B) A purchase agreement may provide that in the event of a dispute regarding the disbursement of the earnest money, the broker will return the money to the purchaser without notice to the parties unless, within two years from the date the earnest money was deposited in the broker's trust or special account, the broker has received one of the following:(1) Written instructions signed by both parties specifying how the money is to be disbursed;(2) Written notice that a court action to resolve the dispute has been filed.

When he sells he doesn't finance 100% of the purchase.

Help him purchase another home Subject to that existing mortgage. 

If you already have the buyer and the seller in agreement, you can probably skip the LOI and go straight to a contract.

On the initial agreement, it clearly stated that the buyer was going to be using financing and listed the terms of their financing

Very simple as Chris notes its just an up front agreement..

You purchased the home as your primary.