Is it obligatory to return money paid as a dowry for a future marriage, despite gifts being bought for the person themselves that exceed the original amount, and given the recipient's financial hardship?

The money a person gave you to keep remains their property, and you must return it to them unless they consent to you keeping it. As for gifts, the giver (other than a parent) is not permitted to take them back, based on the Prophet, peace and blessings be upon him, who said: "The one who takes back his gift is like a dog that returns to its vomit." If the gift was given due to a marriage that did not materialize, some scholars permit taking it back, provided the gift remains in the possession of the recipient and has not changed.