(a) The acceptance, by a creditor, of anything in partial satisfaction of an obligation, reduces the obligation of a surety thereof, in the same measure as that of the principal, but does not otherwise affect it. However, if the surety is liable upon only a portion of an obligation and the principal provides partial satisfaction of the obligation, the principal may designate the portion of the obligation that is to be satisfied.
(b) For purposes of this section and Section 2819, an agreement by a creditor to accept from the principal debtor a sum less than the balance owed on the original obligation, without the prior consent of the surety and without any other change to the underlying agreement between the creditor and principal debtor, shall not exonerate the surety for the lesser sum agreed upon by the creditor and principal debtor.