Have I become an infidel by saying, "Even God cannot help you," as an exaggeration when presenting legal contracts, and what is required of me to do?

This word is an act of disbelief (kufr), and the intention of exaggeration does not justify it. For Allah Almighty is capable of all things: "And Allah is not to be TIREd by anything in the heavens or on earth. Indeed, He is ever Knowing and Competent." (Qur’an 35:44) Repent to Allah with sincere repentance and seek His forgiveness, for He is the Forgiving, the Merciful. He forgives the sins of those who repent, no matter how great they may be. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.' And return [in repentance] to your Lord and submit to Him before the punishment comes upon you; then you will not be helped." (Qur’an 39:53-54) And He, may He be glorified, says: "And whoever does a wrong or wrongs himself but then seeks forgiveness of Allah will find Allah Forgiving and Merciful." (Qur’an 4:110)