Someone asked me to buy some kind of antiques for him, namely copper helmets made in England. I looked for it for him and when I found it, he said: Bring it to me and you will have such and such in return for it. When I went to buy it, I felt that it was not authentic, so I spoke to him on the phone and told him that I would bring it from another province, but he said: Don’t worry, let’s buy it; perhaps it is authentic. When I took it to him, he said that it was authentic and he asked me to bring him more until the number I brought reached seven. After several months, he called me and said to me that he had discovered that they were all counterfeit. I went to the seller from whom I had bought them so that we could return them, but he refused to take back counterfeit goods. Can I give back to the purchaser the money that he gave me, or do I have to do bear the loss with him, or do I do not have to do anything?.
It is permissible for one to buy an item for someone else in return for a specified payment or a percentage of its price, because that comes under the heading of acting as an agent in return for a fee, which is permissible.
It says in Fataawa al-Lajnah al-Daa’imah: (13131): It is permissible for the middleman to accept a payment of a certain percentage of the price for which the product is sold in return for telling customers about it. The middleman may take his payment from the seller or the buyer according to the agreement, so long as there is no unfairness or harm. End quote.
If you had warned your companion about this helmet but he insisted on buying it and asked for more of it, then after that it turned out that it was not authentic, then you do not have to do anything and you are entitled to your payment or the percentage agreed upon, because you were not negligent in doing what was expected of you.
But if you donate or waive part of your profit to help him in his loss, that will be a kind of favour and kindness for which you will be rewarded.
And Allah knows best.