invoicing allowed even after posting invoice reduction

Hi,

In the case of invoice reduction posted for a Service PO, system is still allowing to Post a miro entry for the balance amount, how is this to be restricted.

For Ex. PO price is 100

      and IV is done for 90, then creating another miro entry for the balance 10 rs. is also being allowed, this is be restricted.

please help.

Regards,

Charmaine Martin