A Billing Arrangement can be used to mark slips up or down by fixed amounts and by a calculated percentage. The Client will have a Fees Billing Arrangement section where you can specify either a fixed dollar value, or percentage value adjustment.  Enter negative numbers to decrease the amount, positive numbers to increase. 

Additionally, the bill layout will need to have the Itemized Adjustments Detail section enabled in order for the arrangement message to appear.  You do that by selecting the section, going to the right-hand side of the layout and selecting the Properties tab.  There you will change the Print parameter from Never to Always.

Note any sections with a red border are set to not print on that layout.  This section will lose that red border after being enabled with the step above.