What is the ruling regarding a young man saying about a girl he has not yet contracted marriage with: "She is to me as the back of my mother, and she is divorced by three," and what are the consequences of this in terms of expiation for ẓihār or the occurrence of divorce?

The questioner's action indicates hastiness and foolishness. A person must control themselves and act wisely.

Divorce does not occur except after the marriage contract, and ẓihār (a form of divorce-like pronouncement) applies only to a wife. This is based on Allah Almighty's saying: (O you who have believed, when you marry believing women and then divorce them before you have touched them) and His saying: (And those who pronounce ẓihār from their wives).

Therefore, what the questioner pronounced, whether ẓihār or divorce, upon the betrothed woman with whom he had not yet contracted marriage, is nothing.

If the questioner intended by the ẓihār to refrain from intercourse with her, then he must offer expiation for an oath if he contracts marriage with her and has intercourse with her, as a precaution and to clear his conscience. The expiation for an oath is: freeing a slave, or feeding ten poor persons, or clothing them; and if he finds nothing, then fasting for three consecutive days.