App design help - Contract to Purchase Orders to Invoices

I don't need a full accounting app, but I have a need to keep tabs on my contracts.  Here is the way it works for us, need some help with the design of the app.

We issue a contract for a certain dollar amount.  It may last over several years.  Therefore, don't need to track by fiscal year necessarily.  From a contract, we will issue several Purchase Orders.  I PO is a for a certain amount.  The vendor will then send us invoices.  Every invoice should track to a PO.  So, I want to track a PO by amount and then sum all invoices related to that PO.  On the original Contract, I want to track amount of all PO's issued.  I was originally thinking a Contract table, then a PO table, then an Invoice table.  Is that the right street I am going down?