Ask HN: How can my company participate in someone else's startup?

I have a small company with a few products that's successful enough to pay the bills and the salaries of the 5 people working at it. Recently, someone came to me with the idea of a product that I might be interested in building. What we do best is build products.

This person doesn't have the money to pay for this product to be built and even if they did, that's not what I'm interested in (as in, I don't want to be an agency, I want to own part of the product). I'm having a hard time figuring out how this could work in terms of equity and amount of work. Does anybody have an example of such a thing?

For example, for our own products, sometimes we reach a level in which it's feature complete and we stop adding features while we work on the marketing and if we don't increase revenue for it, we don't pour more development resources into it. When it happens with a third party, I can see this being problematic.

Sometimes one product requires urgent attention and resources are taken from another product to do so. When we own all our products it's a simple prioritization problem of what is more urgent, which product is bigger, etc. When one of the products is partially owned by somebody else, the equation would also include whether we are doing our part of the deal.

Whenever I push this model forward, I see up writing long detailed specs (that are hard to change on the fly) and committing a predefined amount of time to each project that will require time tracking. One of the reasons why I co-founded a startup instead of working on an agency is because I don't like either of those things and I believe they hamper coding productivity, so I don't want to push these things to my developers, that joined this company with the expectation of a different workplace too.