GST Refund matter

my client make Direct export of good outside India in July the said good is detained outside india say UK custom authority before reaching main location due some documents validation expired
we had claimed refund in month of Aug
Good detained outside india never returned back to India the said good destroy at UK Board we have issued credit note against same export in month of March
Now department issue notice to asking pay back excess refund claimed
whether we need pay back excess refund claimed
any case in support of this