When I want to ask someone to do something that has some costs (like personal involvement, time, money), but want to leave it to that person to decide how much to spend on that - how do I express that in a non-contrived way?
Ideally, it should sound incidental, not drawing attention to the decision to make later.
Other: Ok, if you are willing to lend me your tools for free, I will do this project. If I make money, you get part of it.
Me: No, keep it for yourself in the beginning while it is difficult. Later, decide how to pay something back acording to your own ethic assumptions.
Example with implicit solution:
Other: Wow, thank you soo much for that - how can I ever...
Me: Oh, don't thank me - thank it forward by spending some money to one of our care projects.
Here, it's implicit that the person has to choose a project and an amount, and has not to disclose it.