Can a civil contract replace the Sharia contract in marriage, especially if the civil contract includes the essential elements of a Sharia marriage, with the deferral of the determination of the dowry and consummation with the wife to a later time?

The question included two topics: Does a civil contract replace a Sharia-compliant one? And is the contract valid without mentioning the Mahr (dowry)?

A civil contract is permissible if it fulfills the Sharia conditions of a guardian (wali), witnesses, and formula (sigha). Marriage is valid without naming a Mahr, based on the Almighty's saying: "There is no blame upon you if you divorce women before you have touched them or specified for them a obligatory provision" (Al-Baqarah:236). In this case, the woman is entitled to a Mahr al-Mithl (dowry of similar women) upon consummation, and this is called Tafweedh marriage. It is not permissible to agree to waive the Mahr before it has been stipulated.