I called the IRS and was told that I could 1) have the client change the amount or I could, 2) Just report it as 2009 income, or 3) report the lesser amount. I chose option 3. I was told that, at some point in the future, I'll receive a notice from the IRS informing me that I underreported. At that point, I simply have to send a response indicating that I never got the money until 2010, and I use the Cash accounting method. Therefore, I'll claim it in 2010.