Costello forced straight man Abbott to take a cut in pay that then went into Costello’s pocket. While there’s no question that funny man Costello was who people paid to see, their deal from early on was that they were a 50/50 team. People in show business at the time were disgusted by Costello’s treatment of Abbott.
I read somewhere that in the early days of their partnership, the cut was 60-40 in Abbott's favor. And that was rather standard because straight men were considered to be more talented than the comics.
So maybe Lou was just trying to even things out.
From a short biography of Abbott at famousbirthdays.com:
TRIVIA
He (Abbott) was the straight man within the Abbott and Costello comedy duo and was even paid more in the early days of their career because a good straight man was seen as more of a commodity.
I remember reading once that back in the Vaudeville days, the straight man was paid more than the comic. It’s deceptively difficult to be a straight man.