If the plan is 100% to sell the shares back to him and you have a fairly certain idea of how much you want to sell them for and in what time frame, then just sp

If the plan is 100% to sell the shares back to him and you have a fairly certain idea of how much you want to sell them for and in what time frame, then just spell it out that way. The deal is $X up front and $X in 12-18 months. If the business succeeds in 12-18 months then the money to pay you will be there, if it doesn't then the contract will be worth as much as your "shares" (zilch).