Is it permissible to spread deferred payment sales to double capital, with the commodity being sold for more than its original value, and is it permissible to deal with a person who does so, even though the source of his money is unknown?

Selling on credit with a price increase is permissible in Islamic law and does not constitute usury (riba), provided that the seller genuinely owns the commodity and takes possession of it if they had purchased it, then sells it for a known price and on known terms. It is permissible for the buyer to sell it to someone other than the original seller. As for dealing with someone whose source of wealth is unknown, it is permissible, as the 기본 (original principle) is that what is in a Muslim's possession is their property. However, caution should be exercised due to the possibility of fraud and deception.