Is it permissible not to pay the remaining agreed-upon amount to a carpenter who delivered a kitchen and bedroom with quality and design inferior to what was agreed upon, knowing that the price difference for the kitchen is 300 Egyptian Pounds?

What transpired between you is called a manufacturing contract (Istisna'). You both have the right to annul the contract or modify the stipulated conditions and price by mutual consent if the agreed-upon specification becomes impossible to achieve, or to deviate from what was stipulated by your agreement. You do not have the right to refuse to pay the remaining agreed-upon price, but you do have the right to reject what contravenes the agreed-upon specification. You may also waive the remaining price in exchange for the defect that occurred, by mutual consent.