ITC eligibility of 2019-20

During the year 2019-20, purchasing dealer claimed ITC in GSTR-3B on the basis of his books of accounts but the seller did not file GSTR-1, though he included these sales in the turnover in GSTR3-B and paid tax accordingly. Later on in July-24 the Seller filed GSTR-1 for these transcations pertaining to year 2019-20. Now the purchsing party has received a Notice from GST Department on the grounds that ITC claimed in GSTR-3B during the year 2019-20 was not reflecting in GSTR2-A at that time. Is the ITC eligible as per GST Act or not ? pl help.thanks