What is the ruling when a creditor releases one of two debtors from a debt of one thousand, where each was equally liable and jointly guaranteed the other?

Primary text

If a creditor releases one of two debtors from the full thousand, that debtor is released from the principal debt, and his counterpart is released from his guarantee (daman). The remaining liability upon the other debtor is five hundred. This ruling is established upon the principle that the release of one party in a situation involving guaranteed debt extinguishes the guarantee of the other and the corresponding portion of the principal debt.