The current Bill Approvers, Purchase Order Requesters and Administrators of an Organisation can
match not yet approved Bills to Purchase Orders that have been approved but are not
marked as Billed and state the same Currency and Supplier. You'll find more details on matching here:
Bill-to-PO matching.
When you submit a Bill for approval, it will run through the predefined workflow. Approvers in the first step who were also added as Reviewers can
edit Bills during the approval flow. Only when all approvals for a Bill have been collected will it get pushed to Xero in the Awaiting Payment status.
Once all approvals are in place, Bills are pushed to Xero with the Awaiting Payment status and a detailed audit trail of the entire workflow as well as all attached documents.
ApprovalMax also supports a
Batch Payment creation and approval process. All approved Bills in the Awaiting Payment status can be included in a Batch Payment, which will run through the predefined workflow. Once all approvals are in place, the Batch Payment is pushed to Xero and those Bills that have been fully paid change their status to Paid.

Alternatively, you can add a payment to such Bills in Xero to change the Bills' status to Paid. This new status will then get synced to ApprovalMax as described in:
How often is data synced with Xero?