Hi
After getting enough feedback and suggestions you are still asking for enhancement without explaining your complete requirement. I think everyone wants to know that whether this is a third party sale or not. Why don't you come to this third party sale debate instead of looking for some illogical solution?
When you say Invoice in SD forum we assume it customer invoice but in your requirement you are talking about both customer and vendor invoice so I'd like to request you to use complete name i.e vendor invoice or customer invoice.
Thank$