RE: How to restrict specific account to be posted if no job selected on each transaction
There are a couple of approached to this:
You can certainly do this with script but maybe even a workflow (not 100%). Check that the combination is valid and then show a message / prevent the line being saved.
This one is only available with a script. You can add a custom line field to show the list of accounts available, depending on whether job set or not. then you copy that value to the standard field later.
I’d be happy to provide further assistance, depending on your thoughts and level of experience in these areas.