Why does Marcellina claim that Figaro must marry her?

In the opera The Marriage of Figaro, Marcellina claims that Figaro must marry her because he had previously entered into a formal contract to do so in exchange for a loan he received from her.12

Marcellina seeks to enforce this agreement with the help of Dr. Bartolo, who is eager to take revenge on Figaro for past grievances. When the case is brought before the Count, he agrees to judge the matter, hoping to use the debt as a means to prevent Figaro from marrying his fiancée, Susanna. However, the situation is ultimately resolved when it is revealed through a birthmark that Figaro is actually the long-lost son of Marcellina and Bartolo, rendering the marriage contract void.213