Is it permissible not to pay the remaining installment of the agreed-upon amount to the seamstress due to her delay in delivering the garment after the specified period?

The contract between you and the seamstress is a contract for labor hire, and the fundamental principle is the obligation to fulfill the agreement. It is not permissible for the seamstress to delay delivery, nor for you to reduce the agreed-upon amount. In the event that she breaches the delivery time, you may delay payment until she delivers the work. You do not have the right to penalize her by withholding the remaining amount or deducting from it, unless this was a penalty clause stipulated in the contract. If damage arises from the delay and there was no penalty clause, then the matter should be referred to the Sharia court to assess the damage and the penalty.