Incomplete or changed credit sale contracts

(a) When a paper whose contents at the time of signing shows that it is intended to become a credit sale contract is signed while still incomplete in any necessary respect it cannot be enforced until completed, but when it is completed in accordance with authority given it is effective as completed. The burden of establishing that any completion is unauthorized is on the party so asserting. If the completion is unauthorized, and it changes the obligations of either the buyer or the seller in any respect, it is a material alteration. (b) An alteration by the holder of a contract which is both fraudulent and material discharges the buyer unless the buyer assents or is precluded from asserting the defense. No other alteration discharges the buyer, and the contract may be enforced according to its original tenor, or as to incomplete contracts according to authority given. (c) The buyer's written acknowledgment, conforming to the requirements of section 476-7, of delivery of a copy of a contract shall be a rebuttable presumption of such delivery and of compliance with this section in any action or proceeding by or against an assignee of the contract without knowledge to the contrary when the buyer purchases the contract.