What is incumbent upon me for not fulfilling a vow to donate one thousand Riyals from the dowry if I get married?

A conditional vow is disliked, and it has been said to be unlawful (haram). If the vow is made and the condition upon which it was suspended occurs, then it becomes obligatory to fulfill it. Otherwise, nothing is required. A vow is established by a formulation that indicates commitment. So, if the questioner expressed in her vow a formulation by which a vow is established, then she is obliged to fulfill what she vowed when the condition upon which it was suspended occurs. Otherwise, she would be disobedient to Allah the Almighty.