Is it obligatory to return goods purchased through unlawful means as a condition for repentance, given that the goods and their price are lawful, but the method of purchase was unlawful?

It is not permissible to sell an absent item unless it is described in a way that removes ambiguity. Otherwise, the sale is invalid due to lack of knowledge of the item. Some scholars have held the view that the sale of a specific, absent item without a description is valid, and the buyer has the option [to accept or reject] when he sees it. It is not permissible to conduct a sale with a deferred exchange of both countervalues; this is called "selling deferred for deferred." If you purchased an absent item without a description and it is in your possession, rectify the transaction by re-contracting the sale. If the item is gone, or if you engaged in a sale with a deferred exchange of both countervalues and it is not possible to annul or re-contract the agreement, then repentance is sufficient for you.