I believe you're referring to the letting go of Krogstad by Torvald. Torvald is just being promoted to Bank Manager, and one of his first actions is to fire Krogstad. That prompts Krogstad to use the the information he has about Nora to pressure her to convince her husband to give him his job back. It's not just about the loan, but it's about the forging of her father's signature to secure the loan (the note was signed by her father, supposedly, after his death). He threatens to tell Torvald this if she doesn't use her feminine wiles to change Torvald's mind about him. In their conversation about why he is in such a precarious position, because of his immoral action years earlier, he points out that what he did was not so different from Nora's forgery and that the law doesn't much care that a person had noble reasons for the wrongdoing.
The situation is further complicated by Christine's arrival, and Torvald giving Krogstad's position to her. Torvald doesn't want to be seen as weak or indecisive so is unmoved by Nora's requests on Krogstad's behalf. This, then, propels the rest of the action to unfold rather urgently.