Is the buyer sinful if he demanded a reduction in the price of computer screens due to defects in them, and the seller refused to reduce the full agreed-upon amount, so the buyer paid part of the amount without the seller's consent?

Scholars differed regarding selling a commodity with a condition of exemption from defects. The Hanafis permitted it, whether the defect was known or unknown. However, the Malikis, Shafi'is, and Hanbalis held that such a sale is not binding, and the buyer has the right to return the commodity due to an old defect. This is the preponderant opinion.

Therefore, the defects you observed in the devices permit you to return them or demand a reduction in the price due to the defect (arsh al-bay'). The seller must comply with this. If the amount you deducted from the price equals the value of the 'arsh, then there is no blame upon you. Otherwise, you must pay him the remaining amount after deducting the 'arsh. The reference for determining the defect and its value is to experts in the field.

As for the seller swearing that the commodity is worth a higher price while knowing he is lying, this is a "yamin ghamus" (a false oath) that immerses its taker in sin. The proof for this is the saying of the Prophet, peace and blessings be upon him: "There are three types of people whom Allah will not speak to on the Day of Resurrection, nor will He look at them, nor will He purify them, and they will have a painful torment." Among them, he mentioned, "the one who promotes his goods by false oath." Its meaning is promoting a commodity with lies, such as swearing falsely about its purchase price or its quality.