Inventory Supply Details using Rest API

In our solution, we are using CPQ to create customer quotes. Sales Representatives require real-time supply details, including:

On-hand quantities in subinventories where the DFF "Consider Available Stock" is set to "Yes."
Open unreserved Purchase Orders and unreserved Transfer Orders.
These details need to be fetched by passing Item and Organization as parameters.

Currently, to my knowledge:

There is no API available to retrieve unreserved Purchase Order or Transfer Order quantities based on Item and Organization. Could you please confirm or help to provide API which can provide the unserved quantities.