Is the phrase "May God's omen prevail, not yours" permissible?

There is no harm in saying "Fa'al Allah wa la fa'alak" (May Allah's omen befall, not yours). This is a common expression used by someone who hears something displeasing, intending "May Allah be gentle with His servant." It means that a person wishes for good omens and kind words from Allah, without taking to heart what they heard from a pessimistic person.