Assuming you mean big risk, how so? It's very hard to counterfeit modern currency. Color changing ink, watermarks, and the embedded strip are all hard to counterfeit especially in tandem.
Also, on $100 bills, Ben Franklin has a ghost image that can be seen holding up to the light.
But better than all of that is to require the transaction to happen in a branch of your local bank, or post office, and be funded by a money order from the bank or post office.
Some people have visual impairments but they really shouldn't be dealing with large volumes of cash.