Key legal question
Whether an action for restitution of undue payment is admissible when the tax was paid pursuant to a final, uncontested assessment.
Extracted holding
No. A final, uncontested tax assessment constitutes a valid cause for payment, so the payment is not undue.
Extracted reasoning
The admissibility of restitution under the general principle of undue enrichment depends on whether the payment lacked a valid legal basis at the time it was made. Here, the tax assessment had entered into force without challenge, therefore it supplied a valid cause.