(b) That's probably wrong. The situation here is that A and B have misunderstood the terms of their proposed contract. Each party is mistaken about what the other meant by the word 'dollar'. In cases of mutual mistake, the question is whether there has been sufficient agreement to create a binding contract. The test of consensus is objective. The court asks, would a reasonable person, knowing the facts, infer that there was sufficient agreement for there to be a binding contract?
When, because of some ambiguity in the words used, or because the parties are at cross-purposes for some reason, there is no objective 'meeting of the minds' (consensus), then no contract is created: the agreement is legally void.
Applying this objective test to the example, it might be argued that, in most cases, the price of goods sold would be in local currency. But B knew that A was a visitor from New Zealand and in these circumstances the unqualified term 'dollar' is ambiguous. A reasonable person knowing all the facts would probably not be able to infer sufficient agreement for a binding contract.