The non-impairment clause is contained in Section 10, Article III of the Constitution, which provides that no law impairing the obligation of contracts shall be passed. The non-impairment clause is limited in application to laws that derogate from prior acts or contracts by enlarging, abridging or in any manner changing the intention of the parties.[29] There is impairment if a subsequent law changes the terms of a contract between the parties, imposes new conditions, dispenses with those agreed upon or withdraws remedies for the enforcement of the rights of the parties.[30]
http://sc.judiciary.gov.ph/jurisprudence/2011/march2011/172087.htm
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