Praise be to Allaah.
It is permissible to sell an item on credit for a higher price than if it is paid for immediately, whether the payment is to be made in installments or in one payment at the later date. But this is conditional upon the two parties not separating until they have agreed how the transaction is to be done, whether it is to be paid immediately or deferred. In this case the additional amount is not riba, and there is nothing in sharee’ah to specify the amount of the extra payment in the case of deferred payment. But the Messenger of Allaah (peace and blessings of Allaah be upon him) encouraged tolerance in matters of selling, buying, paying off debts and asking others to pay debts.
Fataawa al-Lajnah al-Daa’imah
See Fataawa Islaamiyyah, part 2, p. 335
But if he sells it for ten, for example, then the purchaser subsequently wants to delay payment, it is not permissible for the vendor to add anything to that. If he adds something in return for extending the time period, this is riba and is haraam and a major sin. We ask Allaah to keep us safe and sound.
See also the answer to question no. 1231.