well, since a company handbook is almost never accepted as a contract, any contract would over ride the handbook in almost every situation.
anyway, the CBA would rule unless there was something within the CBA that provided for the rules stated in the handbook to be considered. A handbook is merely a source for general information and generally, neither party using the handbook can be held legally to the statements within a handbook. In a contract, they can.
and the fact you are looking for law or SCOTUS decision leads me to believe this is homework so I'll leave my answer as it is; without citations of support.