that you will indeed have in it whatever you choose?
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
That is, the Book sent down by Allah.
Za'im in Arabic is used for a person who stands a surety on behalf of another, or is a spokesman of others. Thus, the verse means to ask: "Which of you will come forward and say that he has made such and such a covenant with Allah on your behalf?"
That is, "The judgment you are passing concerning yourselves has no basis whatever. It is against reason and you cannot show any such thing written in any Divine Book either. No one can make the claim that he has made Allah promise some such thing, and you cannot make any of your deities vouch that it would take the responsibility of securing Paradise for you from God. How then have you been involved in such a misunderstanding?”