All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, it should be noted that if according to the customs, reducing the specifications of the goods which were required by the first customer necessitates reducing its price, then it is an obligation to agree with him according to the new price so that there would be no deceit in the sale contract as this will invalidate the contract and lead to disputes. For more benefit, please refer to Fatwa 86247.
Furthermore, it is not permissible for you to take 100 from the second customer because he concluded the contract with you based on the agreement of the first customer with you, and it is known that the first customer knows that the price of the goods after reducing its specifications is less than 100 and so he spoke to you privately about reducing the price.
Therefore, it is an obligation on you to sell for the second customer at the same price which you sold to the first customer, i.e. 85, as he agreed to buy with the same price he [first customer] bought.
Al-Hajjaawi from the Hanbali School of jurisprudence, said: “Whoever informs a buyer with a given price and then the buyer comes to know that the price is less, then the buyer has the right to reduce the price [pay the lesser price] and the seller has no choice [to refuse this].”
Allaah Knows best.