If everyone was reasonable and rational then we wouldn't need lawyers, contracts, courts, police,...I guess I have very mixed opinions on this subject!
In one side, I am a firm believer of my word is my bond and I do everything in my power to stand by my word.
On the side, the dealer requires paperwork. I would not under any conditions, accept delivery of any item without a signed copy of paperwork in my hand (signed by me and other party).
In my opinion, when he changed the delivery date, he placed a contract in jeopardy depending on the terms of the contract. Does that mean it is a deal breaker? Depends on why the change.
Personally, I am slow to decide to let go of large sums of money, do those decisions are reached only after much research—I would know why I wanted that machine over another and how important it was. Dollars while always part of these decisions may not have been the primary deciding factor, and thus may mean even with a delay, that is still the machine I prefer, but may not be happy and would let them know it.
From the vendor's perspective there are a lot of deadbeats that think its ok to slough off payments and honouring their end of agreement. What you are describing is a pre-nuptial agreement.