Say, you are being paid piece rate on work you do as a contractor. However your employer set up things in such a way that you are very productive. You feel that you are being overpaid and feel guilty about this.
What to do?
Do not suggest a reduction of your piece rate.
Instead of taking less from your employer, give your employer more.
Invest time at higher quality work even if it slows you down a bit. Do more work on your pieces. Look for ways to optimize even more the workflow in which you are a part. Train other workers to be more productive.