Hi,
For the above scenario, you can use MRP process to create Purchase requisition.
1. MB21 document approval.
2. if sock is not available, your reservation will wait for the stock to receive.
3. Run MRP (Tcode- MD02/ MD04) to create PR for the reservation. MRP will take care all the requirement and stock matching process.
update Source list as relevent to MRP. and select create PR in MD02 transaction.
Hope this will help you in analysing the scenario.
\\ SK