Don
I have used this technique but it has been a few years. I did not have to wait 2-3 months.
I doubt seriously that the mortgage broker or the underwriter knows what an option is or how it works so they are not the best people to ask.
It’s no different that you buying their car, furniture, jewlery, etc…. you can buy anything from the buyer to help them come up with the down payment. Then the buyer can show the “bill of sale” for the option or item which allowed them to come up with the money for the down payment. The bill of sale can state that the funds will be paid AT CLOSING because you sure don’t want to buy an option from someone who does not show up at closing.