A B737-300 FLIGHT CREW WAS GIVEN A BRAKE RELEASE CALL FROM A NON-QUALIFIED SUPERVISOR BECAUSE QUALIFIED PERSONNEL DID NOT COME ON DUTY UNTIL EXACTLY DEPARTURE TIME.

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SCHEDULED DEP FROM GATE AT ZZZ WAS XA00. BAGGAGE HANDLERS FINISHED LOADING BAGS IN THE CARGO PITS AT XA59. AT XB00 SOMEONE ON THE GND CORD TOLD ME TO RELEASE THE BRAKES. I ASSUMED THAT HE WAS THE TUG DRIVER; BUT HE WASN'T USING STANDARD PHRASEOLOGY IN ACCORDANCE WITH FLT MANUAL SOP'S. I HAD TO ASK HIM IF HIS PREDEP CHKS WERE COMPLETE; AND AFTER HIS ACKNOWLEDGEMENT; AND ONCE VERIFIED BY HIM; I RELEASED THE BRAKES. IMMEDIATELY THEREAFTER; I HEARD HIM UNPLUG HIS HEADSET. APPROX 30 SECONDS LATER; THE 'REAL' TUG DRIVER PLUGGED INTO HEADSET. I ASKED HIM WHO THE PREVIOUS INDIVIDUAL WAS; WHICH HE RELIED WAS A SUPVR. BECAUSE OF THE SUPVR'S SINGLE-MINDED CONCERN FOR AN ON-TIME DEP; HE JEOPARDIZED THE SAFETY OF THE AIR CREW; PAX; AND RAMP PERSONNEL. NOT ONLY WAS HE UNQUALIFIED TO ASSUME PUSHBACK DUTIES; HE LEFT THE TUG CTLS UNMANNED FOR AT LEAST 30 SECONDS (WITH ACFT BRAKES RELEASED). AFTER THE ACTUAL TUG DRIVER CHKED IN; HE STATED THAT THE SUPVR MOST LIKELY DID WHAT HE DID BECAUSE THE TUG DRIVER WAS NOT SCHEDULED TO COME ON SHIFT UNTIL XB00. THIS BLATANT DISREGARD FOR SAFETY AND PROCS BY THE SUPVR IS UNACCEPTABLE.