Procurement is most useful in Odoo (OpenERP), here I described some of steps that show your flow.
Here steps that might be useful to you.
Step 1 : Warehouse
– Create New Product
– Update Quantity on Hand
– Set Sale price and cost price
– Create Supplier for that particular product
– Create minimum stock rules for that particular product
Step 2 : Sale Order
– Create one sale order(select product and set related quantity available)
– Confirm order
Step 3 : Warehouse
– Create Delivery order
– check availability
Step 4 : Scheduler
– Run Scheduler
Step 5 : Purchase Order
Now , you can look there is one purchase order created with related quantity.