[1] Quantum meruit is the measure of damages where an express contract is mutually modified by the implied agreement of the parties, or not completed.
The concept of quantum meruit applies in (but is not limited to) the following set of situations: The measure of value set forth in a contract is legally admissible as evidence of the value of the improvements or services but the court (or thus out of court settlement) is not required to use the contract's terms when calculating a quantum meruit award.
Person A (plaintiff in this hypothetical) tells neighbor B (defendant) that he is going to build a wall on their property that will give a benefit to both A and B; A implies that it would be cheaper for both of them if A performs the labor instead of hiring a professional.
This is because there was an implied promise between A and B, which is derived from contract law, because A was acting under the assumption that B would pay for part of his services (see Estoppel).
If a contractor finds part of their work replaced by others through no fault of their own, they seek damages for the amount(s) that the defendant benefited.
A court would decide, following similar precedent, that the promoter is entitled to an assumpsit on a quantum meruit if the promoter has acted in a proportionate way as trustee (depository), delaying forwarding of the principal of the ticket sale part-payments, for sufficiently well-founded premises defects which could have affected its reputation.