OP's grandfather isn't just a grandfather. He's a hoarder. It's not the same as doing him a favour, like mowing the lawn or cleaning out a kitchen cupboard. This is next level shit. As far as I know, hoarders tend to shut people out and are sometimes hostile because they are emotionally connected to their stuff, so the grandfather probably bribed OP to help him. OP probably kept the grandfather at arm's length, but the grandfather wanted help and OP probably saw that as the grandfather's desire to change, so OP helped out, and the cash was just an extra good thing. 'Twas manipulative and mean of the grandfather though, I think. But of course I don't know. Maybe I'm reading into this too much.

His grandfather is a hoarder (assuming OP isn't exaggerating and this is the true state of affairs). Hoarders suffer from mental illness, such as OCD, PTSD, or depression. Taking advantage of his grandfather's mental illness can't be excused.

That isn't taking advantage of his mental illness. From the way it's sounds, OP didn't ask to be paid, the grandfather offered. You're not taking advantage of someone by accepting something they offered you. And I'm sorry but OP deserves to be paid in this case. Hoarders' houses are often filthy to the point of being a health hazard, and dangerous from the piles of stuff. OP gave his time and effort putting himself in a risky situation to help clean up someone else's mess, he deserves to be reimbursed for that, even from family, even if they have a mental illness. The way I see it, the grandfather was the one that took advantage of OP. Both of them went in knowing OP was going to be paid, OP did the work, but he got hurt from his grandfather's mess, and the grandfather refused to pay him. OP did his work and didn't get reimbursed for it when it was already decided on, the grandfather gained something and didn't give in return. That's basically the definition of taking advantage of someone.

Wut.

The post said his grandfather refused to pay him, meaning he was expecting his grandfather to pay for his help. Maybe it's just me, but you help family - especially elders going through a hard time - you don't profit from it.

As multiple others pointed out in comments above, recovering from hoarding is a long and daunting process that requires weeks of work

Still wondering why he wants payment for helping his grandfather...

Because a hoarders situation is dangerous, both from the uncleanliness, and structural integrity of the piles of things, and it takes a lot of time and effort. OP is going out of his way and putting himself at risk by helping his grandfather clean up his own mess. He deserves to get paid for that. This goes beyond just helping, a hoarders situation isn't just a simple chore like mopping the floor or cleaning the bathroom, it's an entire job, and you get paid for a job, even from family.

Not the way I was brought up. Sorry for OP's grandfather.

Just because that's how you were brought up doesn't mean you are right or the better person.

I can see the point that cleaning out OP's grandfather's stuff probably required a lot of work, but if it's someone you're really close to, I'd say helping for the sake of helping is the right thing to do. Just like if OP needed money and his grandfather could have lent him some, he wouldn't be right if he used the opportunity and refused to give OP the money unless OP provided a service. It works both ways. Point is, would you expect the grandfather to pay OP if OP came over to listen to gramps rant about his problems? No, it's just a kind thing to do. Same deal here, sometimes you should just help people without expecting a reward. That said, OP's relationship with his grandfather may be the kind where there's no love or care, just a formality.

#18 If you want self respect it does make him right and the better person. For anyone that can't do something for family without getting paid deserves pity, for they indeed must lead a shallow and selfish existence.