Is the questioner considered sinful for not paying the remaining amount to a dress designer who took an unrequested action, refused to make alterations, and demanded quick payment?

The agreement with the designer to sew a dress is a lease contract, by which the designer is entitled to her wage if she performs the work as requested.

If the designer sews the dress differently from what was requested, she is not entitled to a wage, and she bears the cost of the fabric, and it is necessary to refer back to her.

If she sews the dress as requested, then you are obligated to accept it and pay the full wage. If she sews it contrary to what was requested, then she is not entitled to a wage, and she must pay the cost of the fabric or replace it.

One must be cautious not to withhold the worker's right, for the Prophet, peace and blessings be upon him, said: (Allah the Almighty said: There are three whose adversaries I shall be on the Day of Resurrection: a man who gave a promise in My Name and then broke it; a man who sold a free person and consumed his price; and a man who hired a laborer, benefited fully from his labor, but did not pay him his wage).