Is it permissible to increase the prices because of the enflation?
The former price of the goods that was sold is not frozen. You can sell the goods in its real price. The price of the goods you sell is the price of the day on which you sell it. The former price belongs to the past. You are not bound to sell with that price. However, the increased price must be the current price in the neighborhood. It must not increase to a level that the people in the neighborhood do not find reasonable; it must not be an opportunist or exorbitant price.
In Islam, the price of goods is decided and fixed generally by merchants trading in the neighborhood. It is called the custom of the neighborhood. The price we use should not exceed the price fixed by the merchants in the neighborhood. When the price decreases, you will also decrease it and will have to sell the goods at the price that is settled by the neighborhood.
We must distinguish between the price increase because of the inflation and the increase caused by withdrawing the goods from the market and causing shortage. To increase prices by causing shortage means speculation and profiteering. Speculation, increasing prices by stocking and causing shortage in the market is not permissible. If the shortage is made in a foodstuff, the doer will be created together with the prophet killers in the hereafter.