What is the ruling of Islamic law regarding a merchant who sells oil at a price of 24 dinars per liter, while filling bottles with a capacity of three-quarters of a liter for this price without informing the buyers, and is this act considered usury (riba) or deception (ghish)?

Truthfulness in selling is obligatory and a cause of blessing, while lying and deception are forbidden and erase blessing. If a merchant sells a bottle that is three-quarters full at the price of a liter, knowing this, then this is deception of Muslims and falls under short-changing. What is obligatory is giving sincere advice and being truthful in trade, and avoiding deceit and deception.