Is it Islamically permissible to set two prices for a single product: one for immediate payment (10 Lira) and another for installment payment (20 Lira)? And is the difference between the two prices considered usury (riba)?

There is no harm in setting two prices for a commodity, one for immediate payment and the other for deferred payment, provided that the contract specifies one of these two prices. If the contract is concluded without specification, meaning the buyer accepts one of the two prices without designating it, then the sale is invalid due to the ambiguity that could lead to dispute. Increasing the price in exchange for deferment is permissible in Islamic law and does not constitute usury (riba).