What is the ruling on saying: "May Allah not harm you"? Is it among the verbal prohibitions?

A person's saying "May Allah not harm you" is permissible because Allah is the Giver of benefit and harm, as indicated by Quranic verses.

Similarly, the phrase "Give me, may Allah not humiliate you" is correct, for Allah may humiliate and abase a servant, as Sheikh Ibn 'Uthaymeen pointed out.

However, it is better and more preferable to say "May Allah keep you safe," or "May He protect you," or "May He grant you well-being," out of reverence for Allah Almighty, and to avoid attributing evil to Him, as Ibrahim (peace be upon him) did when he attributed illness to himself, and as the Prophet (peace and blessings be upon him) said: "And evil is not attributed to You."