What is the ruling of Islamic law on purchasing goods for another person with their money, paying the down payment from one's own money, adding a commission to the original price, and guaranteeing the quality of the goods, with the buyer's knowledge and consent?

For the questioner to pay an advance to the supplier from his own money, it is not permissible for this to combine with him being an agent for the merchant in the transaction and profiting from it. This is because paying an advance is considered a loan to the merchant, and it is not permissible to combine a loan with profit. An honest agent is not liable except in cases of transgression or negligence.

If the questioner is compelled to pay the advance and the merchant wishes to pay the full amount, and the commodity is precisely described, then the solution is for the questioner to contract with the supplier for himself, as the buyer. Then, he should conclude a separate contract with the merchant — such as a salam (forward) contract — in which he is the seller and the merchant is the buyer.