Missed invoice of August 2019

Dev (Article Trainee.) (608 Points)

22 December 2020  
What to do with old invoice which was missed in FY 2019-20.
2 invoice issued in August but didn't receive payment till date. but now Party ready to pay invoice amount.
PLZZ suggest..what to do.. should we pay taxes and enter details in GSTR 9 fy 2019-20 directly..
Is ITC available to paying party?