howto custumize M2 for handling variable payments?

Hi WPMU:slight_smile:
I am looking for a solution, that empowers M2 to handle variable membership fees. By default, I can set a fixed price. I want the user to decide this. Once chosen payment carrier and interval would remain as long the membership is valid - only the amount the user shall choose himself. He shall be able to set this during creating the membership. Once he wants to change the amount - admins shall be able to do this on request in M2 backend - who can help?
Appreciating any input...
thx, Rob