Excel custom format for numbers shown as ($K)

I need an Excel custom format to apply K, M, B endings when the original numbers are shown as ($K). For example, $4 is really $4000, and $87.6K is really $87,600,000 or $87.6M. This is common way of improving readability in corporate financial documents where the numbers are all large.

I have an Excel custom format that shows real numbers with their K and M shortened versions:
[>1000000]$#.0,,"M";[>999]$#,"K";$#,##0

I tried making a second version, but it didn't work. It looks like Excel knows what M and K are and won't let me apply them when the source numbers don't agree with the normal definition.