Contracts I Promissory Restitution Restatement Second of Contracts § 86
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A promise made in recognition of a benefit previously received by the promisor from the promisee is binding to the extent necessary to prevent injustice.
A promise is not binding under Subsection (1)
if the promisee conferred the benefit as a gift or for other reasons the promisor has not been unjustly enriched; or
to the extent that its value is disproportionate to the benefit.
Copyright, The American Law Institute
Must have some reasonable expectation of restitution .
Restitution must be for a material benefit, not merely a sentimental one.