Non returning of handset

IMEI no.:911361800307138
Its been a month since I had given my aqua curve for changing broken screen to Intex authorised service Centre. They had promised then that it would take a week n now response is that “Your handset screen replacement is not available with us and hence the phone comes under defected piece category so we can’t return you that handset”.
They are offering a different handset of same model but a used one, n why would I accept a used phone when my phone was only 2 months old.