Here's a possibility for you. The spreadsheet at the DropBox link below will format the cell value as red and will show another cell with a negative number. I'm thinking this does most of what your want but doesn't overwrite the value of the original booking.
It's a matter of
1. Using conditional formatting based on the contents of the adjacent cell and
2. Using an "If" statement to multiply the cell contents by -1 and entering that result into another cell based on the same cell's content.