Billing with different price - Copy Control

Hi Guru,s

My Client has new requirement on pricing please help me in Copy control.

Scenario -  one order is created with 100 qty 90 rs per item and subsequently partial delivery (50 qty) done and billing also done against the 50 qty now requirement is when the product will be available for remaining 50 qty, the billing must be charged on different price (110 rs per item instead of 100 rs per item) but already created order's price should not be changed .. only billing document price should be 110 rs per item.

please help!!

Thanks

Rohtas Sharma