If you are just copying the F8 to Zf8 system will not allow you to maintain pos/neg quantity field in VTFL because the control is in the definition of the F8 the document category "U" .
Where as for the new ZF8 if you change the document category to 5( inter company invoice),here i cant anticipate what would be consequence.
System will allow you to maintain pos/neg quantity field in VTFL.
One more option i would prefer is since your company does not want to bill the other company code,
in the definition of ZIV you remove the Account determination procedure(KOFI00) so that system will allow you to create inter company invoice but no account document generates.
I don't still understand how your company adjust the this billing amount between the companies.
thanks,
Srinu.