I'm a bit confused about the situation... you've been building software, that they need to run their business for free for two years? It sounds to me like you hold all the cards here. If you don't sell it then they have to pay you an ongoing income or rebuild the software from scratch? Do you know how many customers they plan to obtain over the next couple of years? I'm guessing they plan to acquire enough that it's going to be worth their while to spend the money up front. Either that or they've determined there's a huge risk in them not having control of the software (i.e. you could just shut it down on a whim and destroy their business)

Either way... you seem to hold all the cards. What do you want out of the deal? You say you believe in them... why not ask to come on board and get a sizeable share of the company?