From Chapter XI, Promised Gifts in the Third Edition of A Legal Primer on Managing Museum Collections:
A. Obstacles to Enforcement
"If an object (or money) has been promised to a museum and if the gift is not completed, does the museum have any resource? Enforcing such promises is usually difficult because of a lack of what is called 'consideration'. If all the elements of a valid gift (donative intent, delivery, and acceptance) cannot be accepted, then a promise, to be enforceable, must be shown to have the requisites for a valid contract. Contracts require a demonstration of 'consideration' on the part of both parties [...]
To put this another way, for a valid gift one must have the following elements:
1. Evidence of intent to make a gift
2. Delivery by the donor with intent to pass the title
3. Acceptance by the donee (the museum)
If these elements have not taken place and if the donor is unwilling or unable (because of death or change in fortune, for example) to complete the gift, the donee (the museum) has no legal cause of action unless it can prove that the issue is not just an incomplete gift but rather an unfulfilled contract. (A contract is enforceable; a promise of a simple gift, one without consideration, is not.)
To prove a contract, the donee (museum) must show the following elements:
1. A definite promise
2. An agreement between the parties as to what the promise is (a 'meeting of the minds')
3. Consideration on the part of both parties
Consideration is 'an act or forbearance or the promise thereof done or given by one party in return for the act or promise of another'. Both parties to a contract must proffer consideration. In the typical promised-gift situation, the prospective donor has already made a promise to act (the donor's consideration). So to argue that the gift situation is actually a contract, the donee (the museum) must show some form of consideration."
I hope this helps! The only other advice I have is to get your museum's attorney involved before you make any promises to the donors.
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Claire Aldenhuysen
Museum Education Specialist
National Model Aviation Museum, Academy of Model Aeronautics
Muncie IN