The best you can hope for is to collect enough money at the onset of the lease portion of the transaction and structure the payment according to that, first, e.g., a percentage of the total amount of money paid up front, including the first and last month and security deposit, and the option consideration. The balance is outlined in the option portion of the agreement and set to take place over the time agreed to by the parties. That part can be secured by a note on the property for the amount of the commission, but this, like all conditions of these types of agreements, are to be thoroughly discussed and consented to by all concerned, etc. The key question is whether you've earned that portion of the commission payable if and when the "buyer" (optionee) cannot or does not perform. For this reason, the sooner you get paid for services rendered, irrespective of performance in the future, the better.