These Guides might help:
https://forum.manager.io/t/creating-purchase-invoices/7189
https://forum.manager.io/t/spending-money/7821
In double-entry accounting (and therefore in Manager), the purchase order does nothing. It’s only a convenient way of communicating with your supplier and starting the workflow. The purchase invoice for this transaction does the following:
Dr: Expense account (such as Labels)
Cr: Accounts payable
No movement of funds is involved. When you pay the purchase invoice, the Spend money transaction does this:
Dr: Accounts payable
Cr: Cash account from which you pay
At this point, the Accounts payable balance for this supplier is zero, your bank account has decreased, and the expense account has increased.