Is it permissible to pray on a large prayer rug purchased with unlawful money after covering it with a small prayer rug purchased with lawful money?

The one who purchased a prayer rug with ill-gotten money must repent and return the price to its rightful owner if found. Otherwise, they should donate it for the benefit of Muslims or to the poor. As for using the prayer rug and praying on it, that is permissible, because the prohibition relates to the purchaser's liability, not to the object itself.