What is the test for an individual to be liable for unjust enrichment and/or unjust enrichment in a claim against the other party?

"A person can be enriched by merely receiving a benefit. However, the mere fact that a person benefits another is not sufficient to require the other to make restitution. It is well established that unjust enrichment and liability only occur where money or property has been placed in a party's possession such that in equity and good conscience the party should not retain it." Lynch v. Deaconess Med. Ctr., 113 Wash. 2d 162, 165-66 (1989) (internal citations omitted). To be unjust as between the two parties, the party conferring the benefit must not be a volunteer. Id. at 165 (the enrichment of the defendant must be unjust and the plaintiff cannot be a mere volunteer).