Issue with Good Receipt quantity and confirmation quantity

Current:

Confirmation is via CO11N with milestone (need confirmation process by process)

Good receipt is via MB31

No auto good receipt is set because confirmation and good receipt are done by different department.

Now we want to control the quantity of good receipt cannot be more than the quantity of the confirmation.

For this I am exploring the use of user exit and need expert advice on this because I have not done user exit before.

Thanks.