I went to a vendor in the middle of a market in the city center to buy a packet of marrow seeds. I found the price a bit lower than the market price. Therefore, I bought six packets and after a month I bought some packets at the same previous price. I also told a friend of mine, who bought twenty packets and sold them at the market price or at a lower value. After a while, the vendor came to me and said that he had made a mistake, thinking that the packets that I bought contained half a kilo. Actually, there was one kilo in each. I paid him the difference, but my friend had traveled abroad and he is unknown to the vendor, and I did not tell the vendor that my friend bought twenty packets from him.
Please advise and may Allaah reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

Please note that it is valid to sell something that is present before the buyer and seen by him, even if its volume and features are not clarified by the seller, because seeing and specifying an object indicates knowledge of it. Also, the buyer is not responsible for the incorrect estimation of the seller, who may sell for one hundred (dollars) what is equal to one thousand (dollars), even if this is unfair. The majority of scholars are of the view that the seller does not have an option to cancel such a deal, because the evidence of sale and its validity are general and do not differentiate between what is fair and what is unfair.

Consequently, you are not required to pay the difference to the seller, but if you pay it voluntarily, this will be good. Also, you are not obliged to inform your friend because he is not liable under the Sharee‘ah to fulfill this right. If you were to inform him and he paid the difference voluntarily, then this would be fine as well.

Allaah Knows best.