I have a very similar issues.  One of my clients shows a negative almost "payment"  but I cannot clear it as a payment againist the open balance. I also cannot void it or refund it. 

Hi mpb33,

I would need to see all the transactions associated with this invoice.  If you would like to chat further, please email at [email protected]

This may be the result of an issue that can happen when applying/unapplyling cash receipts in AR, Tasks, Adjust receivables. Take a look at KB 81284 to see if perhaps this may have been what happened.  I can't say for sure that's what is going on in this case, but it is a possiblity. You can call support to have an evaluation of the AR data performed to look for potential issues with the data.

This really sounds like issue outlined in KB 81284.  Does it look similar to what I show below in AR, Tasks, Adjust receivables when viewing All Activity?