Loading transactional data before go live

Dear colleagues,

In my actual project we have an unexpected and strange requirement wich is to load all the postings that they have done during the year before go-live in their legacy system (we are going live on may). Therefore we have divided the initial loading in two phases:

1) Classical loading (Balance GL-accounts,Open line items, Assets) at the date 31122008

2)Loading of all postings made during the first half of the year in their legacy system. In that point is when my doubts arise, specially regarding vendor and costumer invoices and their payments. How can we link the invoices and their payments? how can we clear them? and what about the partial payments?

If anybody has done a similar loading i would apreciate his help and guidelines,

Kind regards

Marc Puigvert