What is the ruling on selling products at a price higher than that set by the shop owner, and is the increase permissible for the seller? And what is the ruling regarding credit that was mistakenly charged to another person's phone while the customer had already left the country?

If the owner of the top-up card can be reached, you must give him another card of the same category or return its value to him, unless he waives his right.

If he cannot be reached, you have the choice between keeping the value of the card for him, or giving it as charity on his behalf, provided that if you find him later, you give him the choice between what you did (and the reward will be his), or that you return this value to him (and the reward will be yours).