There isn't a specific field to subtract any administrative fees on a gift record. An administrative or vendor fee can be added as a Gift Attribute or Gift Reference. This allows the donor to receive full credit for the original gift. 

For more information on Gift Attributes and how to add them, please reference How to create a new attribute, create new tables for attributes, and add an attribute to a record in The Raiser's Edge (includes video demo)
For more information on Gift References, please reference What is the Gift Reference field and how is it used?