Finding an empty bowser is the tricky bit, most fuel companies won't mix off loaded fuel with fresh stuff. They say as they have no control over the aircraft tanks, they have to treat any bowser accepting it ass contaminated, and require a clean out before it can be used to fuel other aircraft.
This sort of thing sometimes happens when the aircraft type for the flight is swapped. Seen it twice when a 320 has been replaced by a 319 and the aircraft was refuelled to a predetermined minimum.