Is it permissible to take money from a friend to invest it with a profit share of 30% for him and 70% for me, and the loss is borne by him, with the intention of returning the full capital plus an increase if a loss occurs?

The Mudarabah contract stipulates that it should not include a condition guaranteeing the principal unless there is negligence or transgression. Otherwise, it would be a loan that brings benefit, which is unanimously prohibited. However, if the Mudarib voluntarily guarantees the principal after the contract, without a prior condition or custom, there is no objection to it. Therefore, returning the principal with additional compensation is permissible if it is purely voluntary and not stipulated by the capital owner.