@dc80 

This is only an issue if you're a cash basis taxpayer.  If you're a cash basis taxpayer, you record income and, therefore, the payment date on an invoice, when the payment is "constructively received", that is, made available to you without restriction.  So, in your example, the payment date is January 2, the day you received the check in the mail.  Yes, the client will show that they made the payment on December 26, and issue you a 1099, but the mismatch is not being manipulated, it's what actually happened.  You will have a mismatch between your 1099 and your books, but you do not need to report it as income in December.

If you're an accrual basis taxpayer, your payment received date is mostly irrelevant because you record income as of the invoice date, December 20.  Your client would book the expense as of December 20 if they are accrual or December 26 if they are cash basis.  Therefore, your books and 1099 will match.