A319 pilot reported the aircraft rolled back uncommanded before the tug operator had properly communicated and secured the aircraft.

Narrative

During push back I was instructed to release the brakes before the electric scoop tug operator had fully captured the nose wheel. I felt the aircraft roll back about 18' and heard a soft clunk. I set brakes the and communicated with the tug operator to ascertain the situation. I called maintenance to inspect the airplane. I remediated the tug operator on the correct verbiage on pushback. We pushed about :15 minutes later.Cause: Tug operator omitting the first step on the call outs of 'set brakes' to which the CA replies 'brakes set'. This ground equipment operator went immediately into 'All secure below; FOD check complete; steering disengaged with bypass pin installed; standing by for brake release.......' before they captured the nose tire. 'All secure below' communicates to the pilot; per page X-Y that the guideperson is ready for push back. In this case they were not ready which resulted in this incident.Suggestion is the guidepersons comply with the AOM section X.X page X-Y.