I used to do forbidden things and get paid for them. Now I have repented. Do I have to return the amounts thus earned?
Yes, one who commits haram (forbidden) deeds and accepts payments for them, should get rid of the amounts earned from them when he repents. But such amounts should not be returned to those who paid them.
For instance, when a call-girl repents, she should not return her wages of sin to her customers. When a singer repents he should not return the wages he earned to the band to which he belonged or the party which paid. When a man who used to sell wine or drugs repents he should not return the amounts to those who used to buy from him. Nor should he who used to swear for others as a false witness return the amounts he earned to those for whose benefit he used to swear. The reason being that if he returns the amounts to him who paid, then he will be depositing with him both the unlawful money as well as the unlawful commodity that he took through bribe or wages of sin. Therefore it will be enough if he gets rid of such unlawful earning himself. This is the preference of Sheikh al-Islam Ibn Taimiyyah as well as his pupil Ibn ul-Qayyim as stated in his Madarij.