Is it permissible to say "May Allah not humiliate you" to someone who has done a service for you or to whom you have rendered a service? And is it true that Allah never humiliates His servant, so how can this phrase be uttered?

There is no objection to supplicating with phrases such as: "O Allah, increase us and do not decrease us; honor us and do not disgrace us; give to us and do not deprive us; prefer us and do not prefer others over us; make us pleased and be pleased with us" for oneself and for others. The Prophet, peace and blessings be upon him, supplicated with it for himself and for the believers, and it is considered good speech.