Is it permissible to bid on an item after a potential buyer has already made an offer ('sum')?
Chapter on Selling the Musarrah (Animal with milk retained in udder)
Al-Mughni
Book of Sales
Primary text
Bidding (or making an offer) on an item after another person has made an offer is prohibited if the seller has explicitly expressed satisfaction with the first offer. This prohibition is based on the explicit statement of the Prophet, peace be upon him, reported by Muslim via Abu Hurayrah: "A man shall not bid upon the bid of his brother."
Supporting text
If the seller shows signs of not accepting the first offer, then bidding is not forbidden, as evidenced by the Prophet's sale of possessions in exchange for more than the initial bid. Anas reported that the Prophet asked, "Who will give more than one dirham?" regarding an item bid upon for one dirham, eventually selling it for two dirhams. This practice of auctioning (selling in an escalating manner) is considered an established consensus among Muslims.