Lot specific tracking with the sales invoice before purchase

Hello,

Our customer uses lots nos. with the specific tracking option in order to trace the items sales and purchases. Their usual way is:
1- Purchase order, and pruchase receipt with lot no.
2- Sales order and sales shipment with lot no.

But they have to sell a new special kind of products that force them to start the cycle by the sale:
1- Sales order and sales shipment and sales invoice
2- Purchase order, purchase receipt and purchase invoice.

This is because they don't have all the info they need to make the purchase order before making the sales invoice. They have to wait maybe two weeks for the info of their vendor, and they must send the invoices in advance.

Anyone has a good idea of how to deal with this problem with little developments?

Thank you!