13 GCA § 3106: Sum Certain.

(1) The sum payable is a sum certain even though it is to be paid

(a) With stated interest or by stated installments; or

(b) With stated different rates of interest before and after default or a specified date; or

(c) With a stated discount or addition if paid before or after the date fixed for payment; or

(d) With exchange or less exchange, whether at a fixed rate or at the current rate; or

(e) With costs of collection or an attorney's fee or both upon default.

(2) Nothing in this section shall validate any term which is otherwise illegal.