how to check invoice number before invoice list creation

Hi all,

Thanks for paying attention to my question.  I come up with an  business requirement , that is before invoice list creation, we should make sure no invoice list split happen for those billings should generate in one invoice list. For example, if 5 billings should create  one invoice list, if some one want to generate 4 billing documents and skip the fifth one,  the invoice list should not be failed when we check the invoice number for the combination. 

But I don't know where should be the enhancement point or user exit?

Hope someone can give me suggestions.

Thanks!