A former fed asked a judge to “temper justice with mercy” in sentencing a crooked rabbi facing more than 20 years for a $4 million hedge-fund extortion scheme — saying the holy man was bad for a good reason.

In court papers filed yesterday, ex-Brooklyn US Attorney Alan Vinegrad said Rabbi Milton Balkany’s devotion to the financially strapped Bais Yaakov Jewish girls school in Brooklyn led him to try to shake down billionaire Steve Cohen’s SAC Capital.

“It was wrong, plain and simple. But it was for the school,” Vinegrad wrote.