This really isn't a "Math" skill, but a business skill. I propose it be included as part of the Broker skill. Not a specialty but a feature of it.
Broker
The Broker skill allows a character to negotiate trades and arrange fair deals. It is heavily used when trading (see the Trade chapter).
Negotiating a Deal: Average (8+) Broker check (1D hours, INT).
Finding a Buyer: Average (8+) Broker check (1D hours, SOC).
Managing the books for a company: Average (8+) Broker check (1D hours a day, EDU)
Auditing books to uncover 'discrepancies': Difficult (10+) Broker check (1D x 10 hours, EDU). Alternatively, oppose with an opposed Broker (INT) check.