I'm not 100% sure of what you are asking, but it sounds like you have a person renting booth space from you and you want to take their booth rent off of their sales for the month? are they using your Square account for their sales or do they have their own Square account? If it is yours then you would just transfer less to their bank account when you settle up, but if it is their account there is nothing that you can do to take a fee out off to go to your account. If you give some more details maybe I can figure out exactly what you are trying to do.
As for modifiers, there are not negative modifiers.
