How does one repent from acquiring and spending forged money, and is this act considered theft?

If you are certain that the banknotes are counterfeit, you have sinned by spending them, and you must repent to Allah and give those who received them a replacement in genuine currency or its equivalent value. If you cannot reach them, then give this money in charity on their behalf. If you later find them, they have the choice between approving the charity or taking the money. This action is not considered theft in Islamic law, but it shares with theft the obligation of ensuring the right reaches its owner.