Boss tries to keep former employee on as a contractor for 10hrs/week with slashed pay, doesn't understand why employee is offended

It's nice to be wanted… but there's usually a cost. Especially in the workplace. 

When you leave somewhere, you imagine it's for the better. There is a reason that you moved on and moved out. Maybe you weren't being paid enough. Maybe you wanted to move up, and they weren't letting you. Maybe you just needed a change in your life, and your job was the easiest fix. Whatever motivated you to leave, you made the choice, and you had to sit in it for a while. You had to be okay with that decision, no matter where it took you. The last thing you could have imagined is that they would reach out again. You already moved on, and you were fine to live in that reality. But then you get a message from them: Would you like to do some contract work? Boom. You're back in their orbit.

You probably were picturing a clean break. You left those offices assuming that you would never go back in. You had your last cup of coffee from their Nespresso and assumed that was that. But what if it turned out that they wanted you back? You already gave them the business, so now you have the power. So you do what you do when you have the power, and you ask for more. More money, in this case. You have nothing to lose, right? Nothing to lose but your respect for them.