(b) That's right. What A is suggesting may seem reasonable, but A is definitely not entitled to claim $40,000. The reason is that when the courts calculate what damages to award for breach of contract, they ask: How much must be paid to put the non-defaulting party in the financial position as if the contract had been performed?
In the present case, if B had performed his obligation to pay, A would have received the contract price of $3,500 per tonne, a total of $35,000. What A has tried to claim is damages to put her into the position as if she had never entered the contract at all. But this is not how the courts calculate damages for breach of contract. A would therefore be entitled to claim only $35,000, plus any interest calculated for the period during which payment was late.