i'm using CR XI and MS Office 2007 enterprise.
i'm able to export to excel normally and see my negative amount.
when you export it to excel there are some options there.
1. ms-excel 97-2000 (xls), and
2. ms-excel 97-2000 - data only (xls)
have you try both options?
if you select for option 2, there are additional options on next steps.
one of it is check-box for "export object formatting".
i've tried to check and un-check it, it work fine. i'm able to see the negative sign on my excel.