Likutei Moharan 69:4/ליקוטי מוהר"ן ס״ט:ד׳

14. This is also the concept of betrothal. When something is lacking in wholeness, the Other Side and the forces of evil affect it. The soul, the feminine aspect, is caught among the forces of evil. For she is incessantly dispersed and driven into their midst, as in “Her feet descend to death” (Proverbs 5:5). Silver, or its monetary equivalent, is the abovementioned completion of her light. Therefore, when she sees it in the hand of the man who wants to betroth her and make her whole, she is drawn to him. She yearns for him to complete her light and gather up her dispersed parts from the midst of the evil forces, in order to remove her feet from death.

2And this is the concept of using money to betroth a woman. When her light is completed through the money so that she is not lacking, she is sanctified from impurity and goes free from the evil forces. She is therefore betrothed and becomes his by means of this money, for money completes a woman’s light. This is the connotation of the word KiDduShin (betrothal): she mitKaDdeShet (sanctifies herself) and readies herself to emerge from impurity and receive holiness through him.

3Thus, through all the business activity that a man engages in and profits from, he completes [his wife’s] light and gathers up her dispersed parts. For he must continually complete her light, since he is the aspect of nourishing the tree, the branches and the fruits. His providing them with sustenance and completing them causes the tree to grow branches—i.e., her light to shine—and the branches produce fruits, as explained. This is why immediately following the betrothal he gives her a marriage contract, in which he guarantees: “I will labor, honor, support and provide for you.” He assures her that he will always complete her light by means of the money, as explained above.