(a) General. — A deduction from taxable sales is allowed for bad debts. Any deduction taken that is attributed to bad debts may not include interest or any amount upon which the sales or use tax imposed by this state was not previously paid.
(b) "Bad debt" defined. — The term "bad debt" has the same meaning as when used in the federal definition of "bad debt" in 26 U.S.C. §166 as the basis for calculating bad debt recovery. However, the amount calculated pursuant to 26 U.S.C. §166 is adjusted to exclude:
(1) Financing charges or interest;