You've either witnessed this scenario (or something similar to it) or created it (or something similar to it):
Them: Oh, my goodness! You did such a great job on this. It was way more than I expected. Please take this (insert monetary value here) for all the extra work you did.
You: Oh, no, I couldn't. I am happy to do it free. You don't need to pay me.
Them: I insist. Please. You worked so hard and did so much more than you needed to do. I want to.
You: You don't have to do that.
Them: I know, but I want to.
You: I don't want you to think I did the extra just to make you feel like you should pay me.
Them: I don't think that at all. Please take this. I want you to have it.
You: I couldn't.
You: I don't feel right about taking your money.
Them: You're not taking my money. I'm giving you what I think your time is worth.
You: No, thank you, but I appreciate you being willing to pay me.
Then you walk away thinking you've done something noble, but in the back of your mind you think, “Dang, I could have used that to put a little gas in the car or pay my cellphone bill…” or something like that.
My philosophy is this…
Be a blessing, get a blessing, but don't only seek a blessing while being a blessing.
Regardless of whether your spiritual, religious, worldly, whatever, the universe is cyclical:
Give and it shall be given unto you…
What goes around comes around…
What you sow so shall you reap…
You gotta give to get…
It's all about the karma, dude…
You do dirt, you get dirt…
Bless others so that you might be blessed…
It's not always about you!
You do the work from the goodness of your heart, and the other party wants to compensate you from the goodness of his/her heart.
Let it happen!
They don't need you to tell them they don't have to do it. They already know that, and they've chosen to compensate you in some way for your time, your talent and/or your willingness to be a help to them. Their giving to you will in turn allow them to receive from others. I mean, they've already received your good works, so let them do their good works by you.
Did you ever think that refusing to allow someone else to bless you blocks their blessings, too?