Here is a verdict by Shaykh Ahmed an-Najmi rahimahullah concerning if a person were to do a transaction with another and then come to realize after the transaction has already been binded that the perso who is buying from him is doing something haraam. He mentioned that there is no sin upon the person since he was unaware from the beginning of the transaction.
An example of this is when a man who works by way of fixing cell phones and a person comes to him to fix his phone. Then after he fixes his phone, he comes to realize the phone was stolen.na Here it is permissible for him to return the phone back to him despite it being stolen since the transaction was already completed and what he received for fixing the phone is considered lawful. However, if the person who brought the phone to him is a muslim, then he should advise him about his act of stealing. Allah knows best.
Mentioned And Translated by
AbdulFattaah Bin Uthman