SAPMV45A: User-Exit to handle sub item quantity


In SD sales order (VA01) I create line item 10 with sub item 11 such as:

10	89464988	1 M
11	99912001	1 PC	10

If I change the quantity of main item from 1 to e.g. 2 so the quantity of sub item will also be changed to 2.

Now, Iu2019m looking for a possibility to hold the quantity of sub item to 1.

In SAPMV45A I cannot find an appropriated user-exit for that.

How can I set the quantity to 1 in sub item? What exit can be used for that?

Thank your for your hints in advance.