The big centre half is Wenger's heavy, responsible for rounding up the cash among his team-mates, with fines doubled if they are not settled in seven days.
And Wenger has revealed why he trusts Mertesacker more than anyone to be his banker... because he is German!
Asked why Mertesacker, who has won 85 caps for his country, was his enforcer, Wenger joked: "The Germans do well economically and we respect that. They are the only ones that make money in Europe. That's why we've chosen a German."