The ledger indicates that the person owing the butcher money is none other than Marcellus himself. He appears to be behind on payments for previous orders. Since he's known to be a wealthy businesses owner, this is highly unusual.

They may suspect that Marcellus has secretly been losing money and is hiding it from his partners. Should Marcellus be found connected to the crime via other investigations, this information might grant them access to his vault or financial records. They can collaborate with others to make this happen!