United Airlines probes source of leak about Senator Ted Cruz's travel plans
Cruz rebooked ticket back to US from Cancun on Thursday afternoon
But the senator was initially planning to stay in Cancun through Saturday
That detail was first reported by Skift journalist Edward Russell
Source also revealed Cruz was on standby for upgrade before return flight
Cruz's trip to Cancun sparked uproar as it took place during crisis in Texas
Dozens died and millions were without power after winter storms hit region