Is taking the sum of 1000 dinars from the second customer, "Sade," unlawful (haram)? And is it obligatory to refund him the price difference, following the precedent of the first customer, "Seen"?

It is not permissible to take 1000 dinars from the second customer as the price for the commodity because he contracted based on what the first customer did. It is obligatory to sell it for only 850 dinars, just as it was sold to the first customer. If the seller states a price and then it turns out to be less, the buyer has the right to reduce the excess.