When does Mudarabah begin between two parties according to Sharia: Is it upon the writing of the contract, or when one party says to the other, "Will you be my partner in Mudarabah," and the other party responds affirmatively, or upon the commencement of work?

Mudarabah (definition) is a contract that involves giving a known amount of private capital to a rational individual to trade with, in return for a common share of the profit. It does not become binding merely upon the signing of the contract, but rather upon commencing the work. Both the capital provider and the worker have the right to revoke the contract before work begins.