Any payment on account of the principal balance of a contract which is due on a particular date may be extended or deferred to a later date by mutual agreement, and, upon the amount of the principal balance payment so extended or deferred, a finance charge not exceeding that permitted by § 476-28 upon an original contract in the amount of the principal balance payment, for the actual period of the extension or deferment, may be charged and may be collected in advance at the commencement of the period of extension or deferment or thereafter; provided that the term and conditions of the extension or deferment, including the amount of the principal balance so extended or deferred, and the period of, and the charge for the extension or deferment, shall be set forth in writing and signed in duplicate by the buyer and the seller or holder of the contract, one copy of the same to be given to the buyer and the other copy to be inserted in or attached to the seller’s or holder’s counterpart of this contract.