Purchase returns invoice not mention on GSTR1

we supply materials and issue GST invoice and paid GST
Buyer return the same and issues Purchase return invoice and the same was not mention in GSTR1
they say that we had reversed the ITC against delivery note of purchase return in GSR- 3B

And not required to be shown in GSTR-1 at our end.
Where we show the purchase return invoice and How we Claim ITC charged in purchase return invoice
please help me