And what will you do if "paid" is clicked by mistake?
You can use Manage | Security to make records editable only if a particular expression such as:
PaidStatus = "UnPaid"
Evaluates as True.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
Thanks I will check this out!