Imagine a 5th level fighter, Battle Master archetype.
He uses the Trip Attack maneuver with the first attack of his Attack action and knocks the enemy prone. With his second attack, he hits the enemy with advantage, then as a bonus action (thanks to the Shield Master feat) he shoves the prone enemy 10 feet away into a pit.
Is this allowed by the rules? If an enemy is already prone, can a character then shove that enemy away?