(a) If a party is unjustifiably enriched (enriched party) without any legal ground,
through performance by another, or
in any other manner,
at the expense of another party (disadvantaged party), the enriched party is bound to render restitution to the disadvantaged party.(b) Enrichments which are transferable must be restored in natura. Enrichments which are non-transferable must be restored by paying the sum of money equal to the value of the enrichment to be determined according to the contractually agreed price or market price, including full compensation for the use (usufruct) of the subject matter of the enrichment.