I am having a dispute with Lowes Home Improvemnet

Contracted Lowes to do a complete window install on my home. Work started in October 2022. Ended in May 2023, during which time I had to stuff newspaper into 2 of the windows since they did not fit. Installers were inept, including the fact 3 windows were mis-measured. Told them to stop work and go home. Left hanging, no other contractor available. Finally took my case to corporate, was assigned a senior exec to handle my problem. Now after debating, waiting and them admitting they needed to order window extenders for the mis-sized windows, Lowes is not willing to pay me my out of pocket cost of 2,450.00 for the install. (They had asked for the cost prior to the work being done.) Never intimated they would not pay. They are offering me 1,763.55 and will not tell me how they arrive at the amount except to say they take it all into consideration, including their out of pocket for the window extenders. Excuse me? I am paying for that mistake too? Thank-you for taking the time to read.