Is it permissible to take oil from the press owner before pressing the olives if the quantity of olives to be pressed is small?

Olives are subject to riba (usury), so it is not permissible to sell them for oil due to uncertainty of equivalence. This is because the amount of oil extracted from them is unknown, and equality is a condition for such transactions to avoid riba. Furthermore, it is not permissible to sell an item subject to riba for its original substance from which it is derived, such as olives for oil.