Is it permissible to agree with project funders on a certain percentage of the profit or ownership in exchange for a sum of money, without an accurate calculation of the capital I paid, due to the difficulty of separating it from personal expenses?

The permissible form for selling a part of a company is to sell a known percentage of it for a known amount, with the partners bearing losses in proportion to their share, and profits being distributed according to the agreement. The seller's ignorance of what he spent on the project does not cause harm. The fundamental principle regarding partnerships is permissibility, unless they involve prohibitions, gharar (excessive uncertainty), or jahalah (ignorance) that leads to disputes. Each partner must bear his share of the loss in proportion to his contribution to the capital. A shareholder owns an undivided share of the company's assets proportionate to his shares.