b. Mistake of Law as Basis for Solutio Indebiti

Payment by reason of a mistake in the construction or application of a doubtful or difficult question of law

GR: Solutio indebiti involves only a mistake of fact.

XPN: If the mistake is brought about by the construction or application of a doubtful or difficult question of law. (Sta. Maria, 2017; Art. 2155, NCC)

payer was in doubt whether the debt was due, he may recover if he proves that it was not due (Art. 2156, NCC)

For example, a debtor pays a creditor prematurely because he is not sure whether the debt is already due. The creditor accepts it. The debtor can recover what he has paid prior to the due date of the debt provided that the demand for reimbursement is not made after the debt has become due. (Sta. Maria, 2017)

What is the basis of good faith?

Mistake upon a doubtful/difficult question of law may be the basis of good faith (Art. 526; Kasilag v. Rodriguez, GR No. 46623, 1939)