Project which has potential and is important to me but going to fail because of budget

Question

TL;DR
I'm working on a software project. The whole thing took a while because both parties were somehow unexperienced but now some milestones are finished. The customer is statisfied and wants to go ahead but it may fail because of the budget. The project is somehow important to me (not in a financial way but I don't want it to fail). Now I'm searching for a solution to continue work on it without giving up the money.

The start
My client is a friend of a friend. He knew that I'm studying computer science and could do some coding for him. I agreed and we started to work on the project.

presence
The result of this project is thought to be sold on a subscription base. To find customers for this you'll need something to offer. While there is some offer, it's too less and he will need to pay some more hours to be able to think about selling it to the public.

problem
Although we successfully negotiated a price with which I'm pretty happy (fixed price for every "module") it turned out that there is some requirement of something like ROI. But there won't be one because the program has a good base but not enough to attract customers. This will decrease the motivation to continue and finally could be the project's death.

The reasons for me wanting the project to continue are diverse.
1. It's a greate opportunity to get "real" experience.
2. The project itself is pretty interesting with a possible benefit for people using the result.
3. I'm in the somehow disadvantegous situation of developing own ideas which I'd like to realize but can't (at least in the flexible way I'd like to).

Don't tell me naive. I don't want to give up the money and I'm not going to be exploited.
I'm searching for a way which would make the whole thing a bit more flexible for everyone.
One idea was to negotiate an hourly rate but get paid as soon as there is some income.
This is a bit risky as I can't know if there will be any income in the future.
I'd need some advice what would be a good middle way in keeping the project alive but don't give up money.

Answer 1

As you mentioned, you could do the work and bill with payment sometime in the future (based on the company having revenue). This leaves you taking on the risk of not getting paid, if you go this route I would increase your fee to account for the added risk.

Another option is you could setup a revenue sharing agreement, whereby you would receive a portion of the final revenue the product generates. This makes you a partner in the business, and reflects the investment (in time) and risk (not being paid) you are making in the product. I would go this route if possible, as the potential upside is larger and it sounds like you believe in the end product.