how to restrict ship to organisation -- while creating purchase order to a particualr operating unit

Hi, Friends

How to restrict ship to organisation  to specific operating units only while creating purchase order.

Eg: we have 3 opearting units  Operating Unit-A,  Operating Unit-B, Operating Unit-c

under each operating unit 5 inventory organisations are there....... when i am creating purchasing order in operating unit A at ship to organisation column it is displaying all the inventory organisation where item is assigned.(15 inventory organisation)

But my client requirement is when we select a particular operating unit only those inventory organisation should be displayed which is related to that operating unit

Please guide me