sales and sales return

'x' firm sells goods to 'y' . and record it as sales. but 'y' cancel the bill and return it back without entering purchase in its books of accounts.
on receipt of bill 'x' record it as sales return.
now question is
being the auditor of x and y firm , wat would be your comment . does 'y' firm commited mistake by not recording purchses and purchase return
will IT dept raise any question in both the firm s books accounts
will your ans differ if x andy are sister concern