Explain why krogstad says he would ask for his letter back

The music of the tarantella is heard above, and Mrs. Linde urges Krogstad to be quick. Krogstad now grows suspicious, questioning whether she is saying all of this simply on behalf of Nora. She denies it, and he then offers to take the letter back. She now urges him not to, admitting that this had been her original intention after all. She explains that, since her first discovery of the problem the day before, she has witnessed enough in the house to convince her that Torvald must read the letter.

Why does krogstad ask for his letter back