Are funds obtained through deception from another person – with his apparent consent but with the taker's intention to betray – lawful for the taker?

The relationship between a man and a woman is impermissible except within a legitimate marriage. Mixing in the workplace is not allowed if it leads to something forbidden. As for the money, if he took it on the condition of marriage and then did not fulfill it, you have the right to reclaim it, due to the general saying of the Prophet, peace and blessings be upon him: "Muslims are bound by their conditions in what is permissible." However, if he took it while feigning desire for marriage, without any condition from you, then the money belongs to him, and his action is a forbidden deception.