Sometimes, Purchase Orders and the subsequent Bills do not match. However, there are several solutions you can use to fix this issue:
Allocate the amount only partially
Ignore the fact that Purchase Order and Bill are different. You still have the option to reject or approve the Bill in ApprovalMax
You can edit the Purchase Order in Xero to make it match the Bill (see Is it possible to change a Purchase Order after its final approval?)
Cancel the Purchase Order and create a new one that will match the Bill you have received
You can also issue a new Purchase Order for the price difference in ApprovalMax