Is this particular situation illegal?

I’m taking a math course with a teacher that just switched what textbook they require for the class. It’s $200, you can only get it new, and I didn’t have time to “rent” it online and have it shipped (our bookstore doesn’t rent to students and I live in a rural area without a car, so everything off campus has to be done online).

After buying it and then getting the syllabus in class I realized my teacher would only be having us use 4 out of 15 chapters. At least four other people in the class besides me were very frustrated that we had to buy it because we fall into the huge category of poor students. I decided to scan the chapters and then returned my book, but then I found out that the other 4 wanted the digital copies so they could return theirs, too.

In the past, I’ve had teachers at the same school do this for their students if they’re using a very small portion from a text, so I’m wondering if what I did was ethically, or even legally wrong. My intentions were purely to help my fellow students who were very much in need, and I did not ask, nor did I take any kind of payment for the digital copies I sent them. Also, I don’t plan on giving out any more. It was just for these 4 people and myself.

However, I’m feeling that my idea for creating open-sourced chapters will eventually get me in trouble unless I can back up my case. Any suggestions?

ps. I still don’t have enough money to buy the book. :/