When any taxable article is returned to the seller for adjustment, replacement or exchange under a warranty as to its quality or service and a new article is given free or at a reduced price pursuant to a guarantee, the sales or use tax shall be computed on the actual additional amount, if any, paid to the seller for the new article.
Ga. Comp. R. & Regs. R. 560-12-1-.02
Ga. Code Ann. Secs. 92-3438a, 92-8405, 92-8406, 92-8409, 92-8427.