How to split the supply type to (ON Hand or goods available + PR BUY) when having B2B item in the SO

Summary:

we are going to apply the Back to back buy business flow. but we are facing two un expected behaviors:

• When we have an item enabled as a back to back Buy, and the onhand from the item is 20 Each, when create a sales order with 50 Each we are expecting to have two types of supply ( The onhand 20 each + 30 will be thru the PR ), BUT THE FULL QTY is created a PR , not respecting the current on hand availability.

when check the supply availaibilty UI, Review Supply Availability page is showing "1,000,000,000" for the item and this is not matching with item availability.