Is this act considered a sale upon a Muslim brother's sale, and what should be done about it?

It is forbidden for a person to sell over the sale of his brother, or to bid over the bid of his brother, because that is prohibited. This is like someone saying to a person who bought something during the option period: "Cancel the sale, and I will sell you something better or cheaper," or saying to the seller: "Cancel the sale, and I will buy for more." This action is forbidden.

If bidding over the bid of a brother occurs and the second sale is concluded, it is considered valid by the majority of scholars. However, the Hanbalis invalidated the sale in the case of selling over the sale of one's brother, but validated it in the case of bidding over the bid of one's brother. So, your purchase from the second seller is valid, despite your error in agreeing to the forbidden bidding. If you have not yet purchased from him, it is preferable that you purchase from the first seller, or from another seller other than the second one, to avoid falling into the forbidden and assisting in it.