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.
Reproduced from Islam QA