Conservation of energy doesn't torpedo this idea a priori. The "exhaust" mass would steal energy from the massive body/planet in the gravity assist. If you do

Conservation of energy doesn't torpedo this idea a priori. The "exhaust" mass would steal energy from the massive body/planet in the gravity assist. If you do this a million million times you might slow the planet down to where it has no more energy to usefully donate, but a one-off maneuver is negligible.