Since there seems to be no written agreement, there is likely to be dispute over what was agreed on. The question of substance is when the remainder of the cost must be paid, given this understanding of "holding the puppy until the middle of December", coupled with the $50/month charge thereafter. While it is clear that you have an obligation to pay the remainder of the price, it is not at all evident that "pay the remainder by Dec. 15, or else" is a condition of the agreement. One could reasonably infer that you may render the remainder of the payment in February or later, given the $50/month option (however, that is very cheap, suggesting a misunderstanding). Given the facts as described, saying "I'll be a week or two late with the payment" is not obviously repudiation of the contract.
If the agreement were that you must pay the remainder by Dec 15 and immediately take possession (else pay $50/month for boarding – there has to be some reasonable interpretation of what any "per month" charge would be about), a declaration that you could not make final payment by the specific date could (remotely) constitute anticipatory breach of the contract. But even then, simply being late on the payment does not deprive the other party of most or all of the benefit of the contract (in this case, a sum of money), and is not grounds for terminating the contract. You being late by 2 weeks does not make completion of the contract pointless.
The breeder might maintain that final payment by the 15th was clearly a condition of the agreement, and if you had a written contract with a "time is of the essence" clause, the breeder could prevail. Otherwise, it may be reasonable to think that you agreed to pay $50 for each month (or part thereof) by which you were late, but payment by an exact date is not essential.
The reason to make that payment, even in the current circumstances, is that if you do, you will have fulfilled your part of the bargain, and the breeder would then likewise be required to provide the dog (the performing party i.e. you would be in a strong position to sue). Whereas if you don't make the payment and clearly refuse to ever make the payment, then you will have repudiated the contract and the other party can terminate the contract. They could then sue you for damages, although they might be happy enough with $1400.
Both parties did things that look like repudiating the contract, by putting the puppy back up for sale, and by refusing to ever make the final payment. Both parties can (perhaps) retract their apparent repudiations and the contract can be performed normally, though once the dog is sold, it becomes much more complicated. In this case, consulting with an attorney is clearly the best option. He may look at the facts and tell you that you'll never get your money back, but it may also write a letter which causes you to get a refund, or a dog, and it shouldn't cost $1400.