Update payable
From Protemos Wiki
You can update a payable by pressing the "Update" button on the Finances tab of the related project.
Or you may click on the payable code to open its details page:
It will open the Payable details page.
Press "Update" to display the payable update page:
Make the changes you need and press "Update" to confirm them:
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.