Update payable
From Protemos Wiki
You can update payables by pressing the "Update" button on the Finances tab of the related project.
Or you may click on the receivable code to open the receivable details page:
It will open the Update job page where you can make the necessary amendments:
Press "Update" to confirm changes.
Note: The Language pair and Vendor fields cannot be changed if the job is not in the Draft status, and\or it has payables that are linked to the current vendor.
So in this case, to be able to change the assigned vendor and language pair, you should delete payables and\or change the job status.