Is buying goods in installments through an intermediary who does not own the goods, and who pays on my behalf without my knowledge of the source of his commission (whether it is from my company or from the companies from which I buy), considered a usurious sale?

If the intermediary company pays for the goods on behalf of the buyer and then collects the payment from him in installments with interest, this transaction is prohibited because it is an interest-based loan. However, if the company pays the price and collects it without an increase, there is no harm in that. The other possibility is that the transaction is a murabaha, meaning the company acquires ownership of the goods and then sells them to the buyer for a profit after possessing them, and this is permissible.