Debit Note

Service provider raised an invoice by of 16 lac incl. gst, now due to poor quality of service only 5 lac can be paid out of 16 lac.
supplier would not agree to give credit note of 11 lac.

In my books of accounts already full ITC being taken on 16 lac invoice.
Mainly two issue arise:
1. Passing partly debit note of 11 lac withour cr note from supplier is possible in GST law?
2. Method of ITC reversal.
what can be done at this point? please guide..