Shu'aib bin Abi Hamza شعيب بن أبي حمزة · The Book of Financial Transactions

"Nafi narrated to me from Ibn 'Umar, tht the Messenger of Allah said: 'the two parties to a transaction both have the choice so long as they have not separated, or they have chosen." (Sahih )

"Do not go out to meet the riders, and do not urge someone to cancel a sale he has already agreed upon os as to sell him your own goods, do not artificially inflate prices, and let not a town-dweller sell for a desert-dweller." (Sahih )