Question

Marcelo, a landowner, discovered that his property had been encroached upon by his neighbor, Eduardo. Marcelo and Eduardo entered into a written agreement whereby Eduardo agreed to pay Marcelo a monthly fee for the use of the encroached portion. Over the course of two years, Eduardo paid a total of Php 240,000 to Marcelo.

After two years, Marcelo discovered that the encroachment was actually caused by another neighboring property owner, Roberto. Marcelo filed a case against Roberto and successfully obtained a judgment declaring that Roberto was indeed liable for the encroachment.

Marcelo demanded the return of the Php 240,000 payment from Eduardo on the ground of unjust enrichment. However, Eduardo refused to return the amount, arguing that he paid in good faith, without knowledge of Roberto's involvement in the encroachment.

Discuss the merits of Marcelo's claim for unjust enrichment against Eduardo, citing the relevant provisions of the Civil Code.