How much money should be returned to the shop owner after taking an amount from him without his knowledge seven years ago, considering the currency devaluation, and how can it be returned without informing him? Also, do I need to ask for his forgiveness so that Allah may forgive me?

You must repent to Allah by feeling remorse and resolving not to return to the sin, and by returning the money to its owner, unless he forgives you. Repentance between a servant and Allah means regretting an action, abandoning it immediately, and resolving never to return to it. If the sin involves a financial right, then in addition to that, one must clear their liability by fulfilling and returning the right. If you return the money to its owner, your liability is cleared, and you are not obliged to seek his forgiveness. It is more appropriate to consider the change in the currency's purchasing power, or its value in gold.