Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(Or have ye a covenant) pledges (on oath from Us) on firm oaths from Us (that reacheth to the Day of Judgement, that yours shall be all that ye ordain) that you shall have all that you judge for yourselves in Paradise in the Hereafter?
Or do you have oaths, pledges, binding, secured, on Us until the Day of Resurrection (ilā yawmi’l-qiyāmati is semantically connected to ‘alaynā, ‘on Us’; these words [‘alaynā bālighatun, ‘binding on Us’] contain the sense of an oath [given], in other words, ‘Did We swear to you?’, the response to which is [what follows]) that you will indeed have whatever you decide?, to have for yourselves.
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?”