Am I obligated to pay an additional amount to the seller after a prior agreement on the price of a defective device that was replaced, then the seller informed me after a while of his inability to repair it and demanded an additional amount from me?

Since the defect in the device is a manufacturing defect that predates the contract, the seller is liable for this defect. The buyer has the right to return the device and receive a full refund of the price, or to purchase another device, with the amount paid for the first device being credited towards it. The store is not permitted to deduct any amount from it.