I am not sure if I understand you correctly... Do you mean you have a customer who has purchased a case (worth) of an item, and has the individual units of the case on call off (i.e. you are holding his stock for him) and even though you have sold him the items, you want to show these items in your stock?
Or... Do you mean you have a scenario where you have a customer who has ordered a case (worth) of an item, and you want to ship the items individually if and when he sells one (i.e. you still own the stock and he has a certain number allocated which he can call off one at a time)?