Possession letter not provided by builder

Hi, I bought a flat from DS Max builder in Bommasandra Bangalore. I got the sale deed registration in my name and also have applied for Khata transfer. I had issued them maintenance payment cheque but later found that the builder is taking things for granted and is careless on maintaining the quality. There are so many issues happening in my flat related to quality. Cracks in the wall. Water seepage from ceiling. So I decided to cancel the cheque. Now as I didn't do the the maintenance payment I didn't the possession letter. I am okay to give them their maintenance amount but they have so many quality issues to fix. And I know they will not do it properly so I want them to fix my flat completely and then pay them their maintenance amount. Now after bringing this to their notice they are threatening to take legal action, if the dues are not clear in next 48 hours. I have the sale deed in my name alr. Khata transfer I have applied myself and is in the process. Is their legal threat valid