I heard that if someone doing something for the sake of Allah ONLY
You heard it right.
(Saying): We feed you, for the sake of Allah only. We wish for no reward nor thanks from you; (Qur'an 76:9 More Context)
Another place you might have heard that from,
He who spends his wealth for increase in self-purification, And have in his mind no favour from anyone for which a reward is expected in return, Except only the desire to seek the Countenance of his Lord, the Most High; He surely will be pleased (when he will enter Paradise). (Qur'an 92:18-21)
So, from the above verses it is clear when you do something for the sake of Allah, you do it without expecting any reward nor thanks.
Yeah it is hard to believe and digest, but the truth.
But, in case if you're having financial crisis, and are unable to make enough for your living, then maybe you should focus on earning money through some good means rather than donating either your time or effort or money. (learn more herehere)
May the creator guide us all.