Product allocation

Hi Folks,

I have a scenario wherin i need to allocate product to customer usually prior to arrival and sometimes after arrival, so that the inventory is not shipped to other customers.The allocation happens namely in the scenario described below;

1. Before the purchase department raises the PO the product shd be allocated to designated customers.

2. After the GR when the product is brought to the wharehouse.

3 After the PO is raised and before the goods are received into the wharehouse.

Can you pls tell me ASAP how the scenario can be mapped into SAP functionality

Thanking you.

Rahul