Why was it permissible for the man to say to Moses, "Indeed, I wish to marry you to one of these two daughters of mine," in Surat Al-Qasas, even though one of the conditions for marriage is the specification of the spouses?

The designation of the wife is one of the conditions for the validity of marriage. It is not permissible for a man to say: "I marry you to one of my daughters" without specifying, if he has more than one daughter. The aforementioned verse does not contradict this, as it indicates an offer to marry one of the two daughters, and then the intended daughter was designated later. So the designation occurred at a later stage after the general offer. It was said that Musa (peace be upon him) married Safurah, who was the younger one.