Is it permissible for a seller to demand an increase in the price of an item that has already been sold and paid for, on the grounds of its current price increase, and is there anything in the Sunnah of the Prophet that clarifies the ruling on this issue?

If a person buys something, like grains, and leaves them with the merchant, they become a trust (wadi'ah) with the merchant. The merchant must return them upon request without increasing the price due to inflation. However, if the purchase is described as a deferred item paid for with an immediate price (a salam contract), then the merchant must deliver the sold item on time without any increase beyond the price already received.