It is fine for a man to defer (called muakhkhar) some or all of the dower (mahr) he will gift to his wife for marriage. However, this remains a debt and must be paid regardless of death or divorce. It is forbidden to make a conditional-upon-divorce dower (also called muakhkhar), as some cultures do, to pressure the husband to not divorce his wife.
What the Qur’an Said
“…Give those you have consummated marriage with their due dowries. It is permissible to be mutually gracious regarding the set dowry. Surely Allah is All-Knowing, All-Wise.” An-Nisaa 4:24
What the Scholars Said
Ibn Qudamah: “It is permissible for the dower to be given immediately, or delayed, or some of it may be given immediately and some of it may be delayed, because it is paid in return for something, thus is permissible in this case, like paying for something else.” (Al-Mughni 7:169)
Shaykh Mustafa Umar