Hawzah News Agency – Some things are considered to be recommended when buying and selling, including:

-  one should not discriminate between buyers with respect to the price of goods except when taking into account the buyer’s impoverished situation and suchlike;

-  at the start of business proceedings, one should say the shahādatayn (two testimonies), (1) and at the time of the transaction one should say takbīr;(2)

-  one should give more of what is being sold and take less of what is being bought;

-  if the other party involved in the transaction regrets making the transaction and requests to annul it, one should accept his request.

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(1) That is, testifying to the oneness of Allah and to the prophethood of Prophet Muḥammad (Ṣ).

(2) Takbīr is a proclamation of Allah’s greatness by saying ‘allāhu akbar’.

Source: https://www.sistani.org/english/book/48/