Parashas Metzora · Passuk 15:24
Chabad.org — English translation + Rashi
Torah Temima points to Kiddushin 68a.
English: “And her niddah shall be upon him” — it was taught: From where [do we know] that kiddushin take effect with a niddah? Scripture teaches “and her niddah shall be upon him” — even at the time of her niddah there is [legal] existence/acquisition with respect to her.
Kiddushin with a niddah
Chazal derive that betrothal is effective even during niddah, despite the kares prohibition of relations.