The Condition of the Dowry Has the Most Right to be Fulfilled
Question :
Is it correct to pay a part of the dowry at a later date, because I do not possess the whole sum?
Answer:
It is permissible for a person to marry a woman with a dowry which he specifies, whether it be paid immediately or at a later date. He may also come to an agreement with his wife to delay the whole dowry, or to delay some of it. If they agree to this, and make a contract to that effect, then it is incumbent upon each of them to fulfill the conditions imposed on him or her, according to the words of the Prophet SAW:
"Verily, the condition having most right to be fulfilled is one by which you make the private parts (of the spouse) permissible.
Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 5 Page 303