How can I get previous customer balance in current bill?
Can you expand upon your situation? Are you using Manager for the first time or is it an existing customer which you are trying to amend…
If you are using Manager for the time - I think you need to add any outstanding invoices individually so that when payment is received it can be matched off - refer to http://guides.manager.io/businesses/customers/opening-balances
If you are trying to adjust the aging for an existing customer you could just amend the invoice date.
Without additional information it a bit hard to assist you any further