Is it permissible to give the seamstress an additional sum for her excellent work, even though she did not stipulate it?

Some jurists permit fluctuating ijara (leasing/hiring), which means that the wage is determined based on the nature or timing of the work. For example, saying, "If you sew it today, it's for one dirham, and if you sew it tomorrow, it's for half a dirham," or "If you sew it with normal stitching, you get such and such, and if you sew it with expert stitching, you get such and such." This is permissible because there is no ignorance that prevents delivery, and it is supported by the practice of Umar, may Allah be pleased with him, in his dealings with people regarding sharecropping. However, it must be noted that any additional wage goes to the owner of the shop if the worker is employed by him, and not directly to her.