How to get Contracts Funds Available for Invoicing from REST API

We have an integration process between Oracle and a vendor that creates new billing events on PPM contracts, creates PPM invoices, then creates AR invoices, which get sent back to the vendor.

We need one validation step that ensures we don't try to create a billing event if the remaining available funds on the contract is less than the amount on the incoming billing events for that contract, but we can't figure out how (or even if) we can get this data through the API. Is this possible, and if so how? Thanks!