An Open PO is similar to the regular purchase order payment option in that it is not a collectible payment method. The difference is that the customer has a PO number with a certain amount of funds allocated to it.
To configure this, you will need to go into the admin under the user you want to set this up for and edit their account. Then scroll down to the Open POs section and enter the info like in the screengrab below.
Then under the user's dashboard, they can select which option(s) they'd like to enable (see below). This is an individual setting meaning that you can enable/disable different Open POs for different users.
When the user is checking out and they select the Open POs option, they'll see the available options and the remaining balance.
If the order amount exceeds the allotted amount it will say so under the option: (Not Enough Funds) and even if they select it, it won't let them proceed and will trigger an error message:
If there's an adequate balance in the PO account, the order will go through and the amount will be deducted and reflected accordingly.