Re-adopting Manager is no different from converting from a previous system to Manager. Set your Start Date. As you create your chart of accounts, enter opening balances to match closing balances from the previous use of Manager. For sales or purchase invoices pending at the time of transition, you must enter individual invoices, but with dates before the Start Date. Manager will use any sales or purchase invoices from before the Start Date to establish the opening balances in Accounts receivable and Accounts payable.
https://forum.manager.io/t/setting-up-starting-balances-for-suppliers/7227
https://forum.manager.io/t/setting-up-starting-balances-for-customers/5494