What is the ruling on selling an item at a higher price after its owner has agreed to a lower price?

The increase in the price of a commodity belongs to the principal and is not permissible for the agent, unless the principal willingly grants it to him. So, if the agent buys a sheep for one dinar, then buys two sheep, or buys a sheep worth one dinar for less than that, the increase belongs to the principal. Even what the buyer gifts to the agent at the time of the contract becomes a right of the principal, as it is an increase in the price, appended to the contract.