I guess it depends on the software that you're using but does it not allow you to reallocate the payment?
If not, one option is that you wait until the 'wrong client' pays you and then allocate this against the outstanding balance (and make a note about the mix-up).
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Pearce & Co - Chartered Accountant and Chartered Tax Adviser
Worse case - you could always do additional entries reversing the orginal ones, and then do new ones. If you have the facility put notes on each transaction explain why or referencing to each other.