What is the ruling on someone who said "Hallelujah" believing it meant "Praise be to Allah," and continued saying it despite having doubts, then it became clear to him that it carries a polytheistic meaning?

If it becomes clear to a person that a word has a polytheistic meaning or is impermissible, then he must avoid it. There is no sin upon him for what has passed if he was ignorant of its meaning and thought it was good.