Is it obligatory to inform a friend about the possibility of buying an item at a lower price, knowing that the original seller relies on selling it as a source of livelihood?

If a person asks for your advice, it is obligatory upon you to advise him with what is most beneficial for him. If he does not ask for advice, there is no objection to informing him that the item he desires is available at a lower price, or to bringing it to him. However, allowing people to provide for each other is closer to the objectives of the Sharia. If the two parties to a transaction agree on the price, it is not permissible for anyone else to seek a sale or purchase that would divert from what has been agreed upon, due to the saying of the Prophet, peace and blessings be upon him: "None of you should sell over the sale of his brother, until he buys or gives up." As for before the agreement, it is permissible to seek a sale or purchase.