Is it permissible to demand payment for an educational session after waiving it, thinking it was forgotten, and what is the ruling on the expression "it is for Allah, with my effort and toil in it," and is it permissible to take the money or donate it as charity?

You are permitted to demand rent from your neighbor, as long as you uttered the phrase "it is for Allah" with the intention of donating it and seeking nearness to Allah, and then it became clear, contrary to your assumption, that she had forgotten the rent. You then have the choice to keep the rent or to give it as charity. However, if what you said was merely an inner thought without being uttered, then nothing is incumbent upon it.