Debit Note last year

this happend last year , as i joined newly here we are a contractor, we billed the client as per small purchase orders several times but when measurements taken the value less than tax invoice. so the party passed the debit notes but not sended & not informed us. by now, our debit balance is increasing  and they said we have not that much amount. & we have debit notes raised.

1 .if they raised debit notes it should be reflected in gstr or not ?  ( we did not received any debit notes from them) 

now we want to rectify the Amounts. 

2.  how to decrease the party Amount ?

hoping for quick reply,

Thanks in advance.