Negative rewards balance

Nobody seems to know how to resolve my issue. They keep giving me standard lazy responses like "Your rewards expired and we cannot reactivate them" . That is not even the issue. 

I recently purchases Dell Aurora R8 and financed it through dell preferred account. I was supposed to receive 6% cashback in rewards which I did. I received an email about it and when I opened my account to check, I had a negative Forty (-40$) in rewards balance. Apparently I had negative one hundred and twenty (-120$) something balance and when my recent reward dollars came in they just deducted it from the negative amount. Why should the unused and expired rewards go to negative and why should I lose all my recent rewards? Why don't expired rewards just no show up on my account instead of a negative balance. The responses have been so poor and they close the service ticket without nay notice. I spent more than 2 hours at this point trying to explain what is happening but they don't seem to understand.