What is the ruling concerning money that a brother paid to someone to invest with him for an unspecified monthly profit in value or percentage, with a guarantee of a full refund of the principal upon request?

If a person's field of work is permissible, and they are not guaranteeing the capital, but rather the agreement is to divide the profit according to a commonly agreed-upon percentage, and in case of loss, the owner of the capital bears it alone, then this is a valid and legitimate Mudarabah contract.