Simple answer is would you accept it if it was on a car ,for example.?
You are paying for a new product. You have spotted damage to that product. You have every right to request it be repaired properly. The end of the day its only a new sash
If it was a window I had bought and fitted for my own house, and I damaged it, yes I'd live with it. However you will have paid good money for a new window , and it is damaged, either by the fitter or supplier/manufacturer, then no i wouldn't be overly happy. Options to me are , 1/ builder replaces sash , 2/ you accept it or 3 / you accept it for some money of the job. If it were me I'd be trying for a new sash but would accept money off if offered
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