Customer ledger differences

Currently our Accounts Receivable Ageing Summary shows a difference balance than the related TB balance at our year end. This is for a USD account but the difference remains even when the balances have been revalued to the year end rate. I believe this is due to historic manual journal entries posted many years ago. I would like to clear this historic adjustments with a journal but I am prevented from posting journals to Debtor accounts without a related customer. The individual customer balances are all ok so I don't want to adjust these. Any ideas on how to clear these old manual adjustments?