I can understand your requirement is very common in context of transportation invoices to be linked with Vendor Invoices as a reference point.
There are multiple solutions for the same, based on your current business requirement, you can choose one of them:
1) Generally, transport charges are to be added to the item cost as well, it is not independent of original invoices. We load the charges after posting the vendor invoice through PO, then book a vendor invoice journal to nullify the charges amount (credit side) and book the vendor liability for transport vendor. And put the original Purchase invoice reference in the description. So linkage is through description.
2) To make this process smoother, Microsoft had added the concept of landed costs module, where we prepare a voyage (that can represent a PO or a shipment), and while booking the Purchase invoice it books the original invoice and load the charges amount on the inventory purchased.
Then we create vendor invoice journal and select the voyage (which is linked to PO) to book the Transporter charges. So linkage is through Voyages.
3) In case you find it not worthy to use landed costs module and handling through first process, so instead of using the description fields, you can configure Financial tags in the vendor invoice journal and user can put the PO reference/Vendor Invoice reference in the tag. If you want to have fixed list of PO,then someone needs to create the fixed list for Financial tags and put PO numbers, so that user can select from that only. It will work as reference. So linkage is through Financial tags
Please let us know which approach suits you the most, basis that we can further discuss about that option.
Regards,
Neeraj Kumar
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