Frame-of-mind rule in solutio indebiti

In Uniwide v Titan-Ikeda, the contractor built additional works, but is paid the additional cost of such works. The payor would want to convince the Supreme Court that the payment was a "mistake," and not a "voluntary" fulfillment under Art. 1423 of the Civil Code of the Philippines. Hence, it urges the application of the principle of solution indebiti.
Obligations are civil or natural. Civil obligations give a right of action to compel their performance. Natural obligations, not being based on positive law but on equity and natural law, do not grant a right of action to enforce their performance, but after voluntary fulfillment by the obligor, they authorize the retention of what has been delivered or rendered by reason thereof. Some natural obligations are set forth in the following articles. (Article 1423)
The Court held, however, that it is not enough to prove that the payments made by the payor to the contractor were "not due" because there was no prior authorization or agreement with respect to additional works. There is a further requirement that the payment by the debtor was made either through mistake or under a cloud of doubt. In short, for the provisions on solutio indebiti to apply, there has to be evidence establishing the frame of mind of the payor at the time the payment was made. (G.R. No. 126619)

THE LAW ON SOLUTIO INDEBITI: Article 2154. If something is received when there is no right to demand it, and it was unduly delivered through mistake, the obligation to return it arises. Article 2155. Payment by reason of a mistake in the construction or application of a doubtful or difficult question of law may come within the scope of the preceding article.

Solutio Indebiti.   (Lat.). In Civil Law. The case where one had paid a debt, or done an act or remitted a claim because he thought that he was bound in law to do so, when he was not. In such cases of mistake there is an implied obligation (quasi ex contractu) to pay back the money, etc.; Poll. Contr. 439; Mackeldey, Civ. Law § 458. Bouvier's Law Dictionary, 3rd Revision, 1914, Vol. II, p. 3091.
What is meant by solutio indebiti? Solutio indebiti refers to the juridical relation which arises whenever a person unduly delivers a thing through mistake to another who has no right to demand it.

What are the requisites solutio indebiti? 1. There must be a payment or delivery made by one person to another; 2. The person who made the payment or delivery was under no obligation to do so; and 3. The payment or delivery was made by reason of a mistake.

Solutio indebiti refers to payment by mistake. It is receiving payment by mistake that is not due or does not have such right to demand such payment. It creates an obligation to return such payment. Example: Romeo bought goods from Juliet Store. The goods cost $1500. Romeo gave $2000 to the store cashier and receive a change of $700. Romeo is duty bound to return the excess of $200 to the store. Otherwise, he will be unjustly enriching himself at the expense of Juliet Store.