(a) A retail seller or the holder of a retail installment contract may charge and collect a finance charge in respect of a retail installment transaction and may calculate such finance charge in the manner and at the rate or rates specified in the contract governing the retail installment transaction.
(b) Any sales finance company may purchase or acquire or agree to purchase or acquire from any seller any contract on such terms and conditions as may be agreed upon between them. Filing of the assignment, notice to the buyer of the assignment, and any requirement that the holder maintain dominion over the payments or the motor vehicle if repossessed shall not be necessary to the validity of a written assignment of a contract as against creditors, subsequent purchasers, pledgees, mortgagees and lien claimants of the seller. Unless the buyer has notice of the assignment of the contract, payment thereunder made by the buyer to the last-known holder of such contract shall be binding upon all subsequent holders.