Purchase Order going low on Funds(Best Solution?)

Hi,

What would be the best solution to this user requirement. Please note we are not using Encumbrance accounting or Budgetary Control.

One of the main things we are trying to solve is
how to get end users(requestors) to better manage their PO balances as a
majority of the invoices on hold are a result PO’s running out of funds.
If requestors(end-users) were receiving auto generated alerts that their PO’s
were running low or out of funds they could “maybe” proactively submit change
requests adding funds to the PO’s and thus stopping invoices from going on