I am mildly pumped today as a customer of mine just got a refund from the IRS today of penalties on a debt he paid off in full. Our agreement was that the customer would give Washington Tax Services 30% of the proceeds of what we got back. We call it a "penalty abatement" done on contingency.
The way that we protect ourselves is the customer signs a Power of Attorney and agrees that all refund checks from the IRS go to us first. When we get the check, we scan it to the customer and they pay us the 30% and then we release the check.
These are fun cases, of course, and we require only $150 as a downpayment to start the case.
Other penalty abatement cases described here: Articles on Penalty Abatement