When you did this last year (2006) with a 2007 date, how did you balance your bank account?  When you did a bank rec for September 2006, the check that paid those invoices would have a post date of 2007 and would not show up in the Trial Balance for as a debit to the cash account and a credit to Accounts Receivable unitl September of 2007.  I would think that A). Your bank statement would not reconcile to the G/L account and B) The invoice amounts would still be reflected in the A/R G/L account balance in the TB and would not reconcile to the A/R Aging.
 
I am backing up my data files because I want to try it on my files to see the results.
 
You should be able to reverse the payment in AR-C
Enter the Inv # or the Customer Code
Enter the Date (the wrong date, 2007 date)
Enter the amount as a negative
Enter the check number
Enter the Deposit number
Select "Reversal"
Make sure you select the correct bank account, if you have more than one.
 
Now, Enter the payment correctly.
 
I just tried to enter a payment with a date greater than 12/31/07 and it would not accept it.
I can go back in time and enter payments, but I can't go foward beyond 2007.
 
We get entries here where users transpose dates when entering all kinds of items.  The main way you discover these mistakes is IF you reconcile all your registers on a monthly basis.
 
BTW, back up your data before you try my suggested fix.
 
 

The method I suggested should still work to back out the payment and repost to the correct date.
 
I just don't know how you posted a future payment, unless you physically booked the payment in the year 2007 and you had advanced the fiscal year to 2007 and then it's not really a future fiscal period.

glad to have returned the favor as you once responded to my cry for help.