You can increase your bounty using this tool. It works directly and does not use save-file.  When prompted for "Enter bounty value to search for": You should enter Bounty of one of your car. It is important that you do not enter total bounty, but that related to one of your car. The tool will search for that value and ask you new bounty value. After entering new bounty value, your bounty for that car will be changed to this new value and hence total bounty also will be changed.