A dose of reality
A very long time ago, a friend of mine let me work in his business while I was waiting for the government to process my paperwork to teach.
I was happy, he was happy, his customers were happy.
After I'd been there for two months, an acquaintance told me I was being radically underpaid and badly used.
I went in to work the next morning and something in my body language told the boss - my friend - I was unhappy. He asked what was up.
I told him. He showed me the books. I realized that I was damn lucky, he was paying me what I eventually became worth and not what I was worth at the beginning.
These ungrateful assholes have presumed upon your kindness.
Your answer was just right. If you're lucky, they will now quit. I'll bet you anything you like, they're cheating you where ever and when ever they can, including sabotage.