The 21-year-old defender joined Posh in January, scoring once in six matches.
But Posh manager Grant McCann revealed that Ball was left out of the team to face Rochdale on Saturday because "he didn't agree with the team picked".
Ball is back at the Millers, but will be unable to play as he is still registered by the League One club.
Rotherham United have confirmed Dominic Ball has returned from his loan at Peterborough United because of a "disagreement over selection policy".