What is the legitimate Islamic method for returning the share of a partner who withdrew from a company that was harmed by the Coronavirus, with an agreement to postpone part of his dues and for him to bear part of the loss if it occurs?

The scenario mentioned in the question, regarding the purchase of a partner's share with a portion of the payment made immediately and the remaining portion deferred until the goods are sold, with the possibility of the amount decreasing in case of loss, is not permissible Islamically. The permissible Islamic solution is to purchase the partner's share for a known and specified price that is not subject to increase or decrease, regardless of whether the goods profit or incur a loss. Furthermore, the remaining deferred payment must be for a known and specified term, not contingent upon the sale of the goods. It is permissible for the remaining partners collectively, or some of them, or one of them, to buy the partner's share.