Praise be to Allah.
It is not permissible for a worker in the shop to sell anything on his own account and keep the profits for himself, because this is a betrayal of trust and is deceiving the owner of the shop. We praise Allah, may He be exalted, for guiding and enabling you to repent and ask what you should do now.
You must return to the owner of the shop the profits on the cards that you sold, and you should strive hard to work out how much these profits were, based on what you think is most likely to be the case, so that you can absolve yourself.
With regard to the amount that you say you left in the shop – which is 150 pounds – if you intended it to be compensation for the amount that you earned, then you can deduct that from the total amount that you owe.
With regard to the amount that you gave in charity on behalf of the owner of the shop – which is 140 pounds – you do not have the right to deduct it from the total amount that you earned, which is now a debt that you owe, because what you are required to do is return the money to its owner, not give it in charity on his behalf. It is only permissible for you to do that if you have lost contact with this man. But since you know where he is, you do not have the right to give anything that you owe in charity on behalf of the one to whom you owe it.
You should understand that this charity will be regarded as having been given on your own behalf, if Allah wills, and we pray to Allah that you will find it in the balance of your good deeds.
You have to work out what remains due to the shop owner and send it to him: in order for your repentance to be valid and acceptable, if Allah wills.
The changes in the exchange rate from what they were at that time do not affect you. All you have to do is to return the money that you took from the profits, without any increase.
We ask Allah, may He be exalted, to accept your repentance.
And Allah knows best.