Is it permissible not to pay the price of a commodity worth 390 Egyptian pounds, in exchange for a remaining amount of 460 Egyptian pounds owed to the questioner by the merchant who did not inform him of the true price of the first commodity?

If you purchased an item, being unaware of its usual price, and you were not negligent by rushing into the purchase, then some scholars have stated that you have the right to annul the sale if the price difference is excessive. However, you do not have the right to compel the seller to return the price difference; rather, you either annul the sale or keep the item and do not reclaim any part of the price.

Some scholars have stated that the buyer may keep the item and take the difference between the price paid and the usual price.

However, since the seller reconciled with you by offering half the difference between the two prices, and you accepted and took it, you are not entitled to revoke this reconciliation. Therefore, you may not refuse to pay for the item; otherwise, that would be consuming wealth unjustly.