Why didn't you say something when you saw the money paid out in your check? The supervisor messed up sure, but instead of correcting it you took the money and hoped for the best.
Did the sup just mess up?
We have had sups in our bldg intentionally code sick calls as vacation days without the driver's permission, and they have admitted to doing so. One can only guess at the motivation...
-Retaliation?
-Killing off individual vacation days because they are hard to cover?
-Avoiding a black mark on the sups record for too many sick calls?
-Paying vacation days before the pay raise went into effect?
The OP has indicated that was done to retaliate for a grievance, which would be hard to prove, but certainly not unprecedented.
Even if it was a simple error on the sup's part, the driver should not have to lose his scheduled time off because of the sup's error.
The OP should make sure that if the sick days/vacation days were paid before our latest raise, he demands that UPS pay him the difference between the pay rates for the 4 days he was paid, and the 4 days he was scheduled off.