When a transaction is recorded in the subsidiary book, it is posted to ledger by posting individual entries to personal accounts and the total to the relevant nominal account. Now if the total of the subsidiary book is posted to ledger but one (or more) of the individual entries are not posted from the subsidiary book to the ledger, the trial balance will clearly go out of agreement as total debits will not be equal to total credits.
A credit note for $2,160 received from Melbourne Wholesalers was recorded in the Returns Outwards Book but not posted to their personal account in ledger. It follows that Melbourne Wholesalers Account needs to be debited by $2,160 to place the entry in that account. The corresponding credit will be placed in the Suspense Account.