Why did krogstad send the document back to nora?

Krogstad sent the document back to Nora because he wanted to manipulate the situation to his advantage. By returning the forged document, he aimed to leverage it as a means to coerce Nora into persuading her husband, Torvald, to retain him at the bank. Krogstad's actions reflect his desperation and desire for job security, ultimately intertwining with Nora's own struggles and secrets.