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You are talking about offer and acceptance, which is a contractual doctrine.
If an offer is made and an acceptance is made, then a contract is formed.
If an offer is made and a counteroffer is made back, then there is no contract.
However what has happened is that the buyer sent a counteroffer again, and then your wife replied and made an acceptance.
If so, then there is (arguably) a binding contract. This is because in NH, the Uniform Electronic Transaction Act has been adapted. Under the Act, a record or signature may be seen as binding. By writing back "we accept these terms," she essentially made an acceptance. Her failure to see the attached contingency does not change that fact and is equivalent of a person failing to read the contract before signing it - she would still be bound.
Someone in your situation can TRY to get out of the contract by arguing that she could not have been expected to see the attachments as part of the contract. If this can be proven, it can be argued that there was no "meeting of the minds" and thus, no contract formed. (A valid, enforceable contract requires offer, acceptance, consideration, and a meeting of the minds. Tsiatsios v. Tsiatsios, 140 N.H. 173, 178 (1995)). However in this day and age, it is pretty obvious to most people that attachments to email are part of the email. Ergo, this would be a weak affirmative defense if the buyer tries to force the contract.
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