1. You cannot post on invoice more items than you received. This is standard process. If you have this case, you must to do next:
a) post receipt for 15 items to standard location
b) post receipt for remaining 5 items to other location (e.g. some OnHold location)
c) post invoice for all 20 items
d) post credit-memo for 5 items
2. Vendors invoice and your NAV posting invoice must to be the same (in penny). But, NAV may have different rounding method from your vendor software. Because of thet, you can open statistic and do next:
a) if you have different VAT, you can change VAT Amount for each VAT% in VAT subform
b) if you have different Total Excl. VAT, you can change this amount on statistic header in Total Excl. VAT field.
All of them are standard procedures of posting. They exists on all NAV versions.