Relationship between ancumbrance and pay items on complex services purchase order

Hi. Is there any better way to solve the following situation:

1. Requester created purchase requisition for service with assumption that service will be realized across 2 years (2 distributions, 40% in year 2013 and 60% in year 2014)

2. Buyer created RFQ based in requisition, entered and awarded quote.
We can assume at this point that requester's prediction was correct and supplier really offered 2 pay items for complex service

3. Buyer created Complex Services PO based on awarded quotation and automaticaly is created 1 line-1 pay item-2 distributions (mapped to purchase requisition)

4. In order to have correct milestones as agreed with supplier, buyer splits one pay item into two pay items (40% in year 2013 and 60% in 2014).