Don't do it for the promise of their future activities. There is a chance that they might never deliver (you're the only one in a position to estimate this).

Think of it in terms of an equity investment. If they invest $X on a $Y premoney valuation, they own X/(X+Y) of the company. After the investment, the company then pays partners based on contributions (e.g. it pays you a salary and pays them sales commissions based on success - or any other means you choose.).