Not knowing what your data looks like, it's hard to fully answer your question; I'd recommend posting a packaged workbook. However, I do not see why a calculated field wouldn't work. Have you tried something like:
if [Account Num] = 123456 then abs([Account Value]) * -1 else [Account Value] end
Then you use that calculated field instead of your database field.
And, as an aside, it seems odd that you want to selectively negate certain values based on account number. Perhaps a better solution exists upstream of Tableau.