Section 15-105. Discharge of one obligor, without reservation

1. If an
  obligee releasing or discharging an obligor without express  reservation
  of  rights  against  a co-obligor, then knows or has reason to know that
  the obligor released or discharged did not pay so much of the  claim  as
  he  was  bound  by his contract or relation with that co-obligor to pay,
  the obligee's claim against that co-obligor shall be  satisfied  to  the
  amount which the obligee knew or had reason to know that the released or
  discharged obligor was bound to such co-obligor to pay.
    2.  If  an obligee so releasing or discharging an obligor has not then
  such knowledge or reason  to  know,  the  obligee's  claim  against  the
  co-obligor  shall  be  satisfied  to  the  extent  of  the lesser of two
  amounts, namely (a) the amount of the fractional share  of  the  obligor
  released or discharged, or (b) the amount that such obligor was bound by
  his contract or relation with the co-obligor to pay.