This issue may occur when the revenue has been previously adjusted, and the adjustment post date from that adjustment does not match the deposit post date for the revenue.  

To resolve this issue:

1.  Click "Edit posted payment" and change the Adjustment post date to match the deposit post date.  Save this change.
2.  Click "Edit posted payment" again and change the Adjustment post date back to what it was originally.  Click "Yes" to the prompt to unlink the payment from the deposit.  Save this change.
3.  Click "Edit posted payment" again and make the desired adjustments to the payment.