Fill out Field from Related Record

I am trying to script what seems easy at first (and actually may be)

I have a field on our transactions called "Tracking Number"  This is the tracking number for that particular line of a transaction.  We special order every item so there is always a purchase order associated with inventory items on Sales Transactions.  What I would like to do is... When our purchasing purchase recives a line on a purchase order I would like them to place the tracking number from the shipment on that receipt.  I would then like this to back fill all the way to the associated sales order.  Since the purchase order is created from the Sales Order