I have a customization in AX 2009 that requires reversing a specific packing slip?

Example, I have a sales order (SO-001) then I have posted 3 packing slips:

PKS-001 = QTY: 5

PKS-002 = QTY:3

PKS-003 = QTY:4

 Total Ordered Quantity for the sales order would be 12.

Then suddenly, there is a need to change the quantity of PKS-002. So instead of 3, let's say I reverse the quantity and set it to 2,. Therefore, The total ordered quantity for the sales order should be recalculated and the posted quantity for PKS-002 should be changed to 2.

 I noticed that in AX, when you reverse quanity, by putting a negative quantity value in the Deliver Now column of the Sales Order, then post it again as a Packing Slip, I noticed that it always modifies the first packing slip that has been made (FIFO). As the customization requires, the users should have the liberty to select what packing slip to reverse..

Any ideas are welcome! I hope that you can help me with this problem.