Section 17. The holder of an instalment sale contract may, upon agreement with the buyer, defer the scheduled payment of all or part of any unpaid instalment payment or payments, or renew the unpaid total of payments of such contract. The agreement for such deferment or renewal shall be in writing and signed by the parties thereto.

For such renewal the holder may contract for and collect a refinance charge. Such refinance charge shall be computed on the amount obtained by adding to the unpaid total of payments of the contract the insurance cost incidental to refinancing, and by deducting any rebate computed in accordance with the provisions of section sixteen which may be due to the buyer for prepayment incidental to refinancing, at the rate of the finance charge in the original contract, for the term of the renewal contract, and subject to the provisions of this chapter governing computation of the original finance charge; provided, however, that the holder shall not be allowed the acquisition cost of twelve dollars and fifty cents.

For such deferment the holder may contract for and collect a deferment charge. Such deferment charge shall not exceed an amount equal to one per cent per month simple interest on the amount of the instalment or instalments, or part thereof, deferred for the period of deferment. Such period shall not exceed the period from the date when such deferred instalment or instalments, or part thereof, would have been payable in the absence of such deferment, to the date when such instalment or instalments, or part thereof, are made payable under the agreement for deferment; except that a minimum charge of one dollar for the period of deferment may be made in any case where the deferment charge, when computed at such rate, amounts to less than one dollar. Such agreement may also provide for the payment by the buyer of the additional cost to the holder of the contract of premiums for continuing in force, until the end of such period of deferment, any insurance coverages provided for in the contract. No deferment charge shall be made on any instalment for which a default charge has been made unless the default charge on such instalment is rebated in full. If a contract is prepaid in full during a deferment period the buyer shall receive, in addition to the refund required under section sixteen, the refund of that portion of the deferment charge applicable to any unexpired months of the deferment period.