Must the profits from selling "phone cards" that were purchased with private money be returned to the employer, bearing in mind that 10 years have passed since the incident, and the employer is extravagant in sins? And should the amount be returned in his name or given as charity on his behalf, and how should the value of the profit and the difference in the pound's exchange rate be estimated?

It is not permissible for a worker to sell items for their own benefit and keep the profit, as this constitutes a breach of trust and cheating the employer. The profits from the cards that were sold must be returned to the shop owner, and the profits should be estimated based on what is most probable. If the amount left in the shop (150 pounds) is a compensation, then it should be deducted from the total debt. As for the amount spent on behalf of the shop owner (140 pounds), it is not permissible to deduct it, because what is obligatory is to return the money to its owner, not to give it as charity, as long as the owner can be reached. This charity would count for you. You must estimate what is owed to the shop owner and send it to him for your repentance to be valid. The difference in currency exchange rates does not matter, as what is required is to return the money taken from the profits without any increase.