Split/modify an approved PO

Hi
We raised a PO for procurement of software
The rate entered in the PO per unit is inclusive of the license cost per user and also support for a year
E.g.
PO raised as follows
10 licenses @ 100=1000
20 license @ 50= 1000
Total=2000

The vendor sends 2 invoices as follwos
Invoice 1
------------
10 licenses @ 80=800 (license cost per user)
20 licenses @ 20=400
Total = 1200

Invoice 2
-------------
10 licenses support@20=200
20 licenses support@30=600
Total = 800

Questions
1. How should these 2 invoices be matched to 1 PO
2. Is there a way to split or modify the PO. The PO has been approved

3. The user without checking all these details enters the invoice and starts matching and realises the above fact .