All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
The khiṭbah (engagement) is a promise to marry and an expression of initial acceptance, but it is not considered the formation of the marriage contract itself — since the contract has its own specific pillars and conditions, among them: ījāb wa qabūl (the offer and acceptance) using specific wording, the presence of the walī (guardian), and the presence of witnesses. This is affirmed in the Personal Status Law No. (15) of the year (2019), Article (3), which states: "Marriage is not concluded by the engagement, nor by a promise, nor by the recitation of al-Fātiḥah, nor by the receipt of anything toward the mahr, nor by the acceptance of a gift."
Based on what appears from the matter presented, the man gave his fiancée a sum of money toward the mahr, as well as an additional sum which he asked her to spend for the purpose of her travel to him to complete the marriage contract. Articles (4/B), (4/C), and (4/D) of the Personal Status Law address the disposition of funds and items paid toward the mahr under various scenarios in which the engagement comes to an end:
Article (4/B) states: "If either party withdraws from the engagement, or it ends due to death, then the suitor (al-khāṭib) or his heirs have the right to reclaim what was paid toward the mahr — whether in cash or in kind, if it still exists in its original form — or its value on the day it was received, if returning the exact item or its equivalent is not possible."
Article (4/C) states: "If the fiancée has used what she received toward the mahr, or part of it, to purchase items for her trousseau (jahāz), she has the choice between returning what she received, or handing over what she purchased of the trousseau — in whole or in part — if the withdrawal was on the part of the suitor. Her right to this choice is forfeited if the withdrawal was on her part."
Article (4/D) states: "If the engagement ends due to death, or due to an unforeseen circumstance beyond either party's control that prevented the marriage contract from being concluded, then none of the gifts are to be returned."
Accordingly: the fiancée must return what she received toward the mahr — unless the heirs choose to forgive/waive it— but she is not required to return the expenses she incurred related to her travel undertaken to complete the marriage contract, since this was done at the request of the suitor. And Allah, the Exalted, knows best.