If you know which field to set, you can use plain old Set Field .
• Go to Layout ( CirculationDesk )
• Set Variable [ $patronID ; <yourFieldName> ]
• Go to Layout [ original layout ]
• Set Field [ Master::patronID ; $patronID ]
From your other post (scripting), I have the suspicion that some (all?) of your layouts are based on the same TO.
If the CheckOut layout is based on the same TO as CirculationDesk, then the active record is the same
on both layouts, since layouts with the same TO have the same found set, and switching from one to the
other doesn't change the active record (or the found set).
In this case you wouldn't even have to change layouts, but then I would think your data model is somewhat flawed.
I gather this is a library database. Are your books, patrons, checkouts etc. all in the same table?