Does selling a commodity at its pre-discount price, without informing the buyer of the discount it has received, make the money generated from the sale unlawful, and what should be done in this case?

The sister bought the bag for her sister at her request, so she is her agent in this transaction. The discount is for the sister who requested it, not for the purchasing sister. An agent may only act within the scope of the authority granted to them. If the increase were rightfully hers, the Prophet, peace and blessings be upon him, would have paid it to her. It is obligatory to inform her of this. If she permits taking the difference, then there is no sin. Otherwise, it must be returned. Informing her of a price other than the true price in order to take the difference for oneself is forbidden lying and consuming wealth unjustly. The true price is the price after the discount. If the sister wishes to profit, she can either request a commission for that service or buy it for herself and sell it to her sister at a price they both agree upon.