Here's what likely happened:
When you placed the order, Apple authorized your PayPal card for $1700 to make sure the funds were available and the card went through, but your card was never actually charged. Yet a hold will deduct from your available balance. This is standard operating procedure for any debit or credit card. When they shipped your iMac, they converted that authorization to your charge. However, I think because PayPal is incompetent, because I've seen this before on my PayPal account, when a merchant converts a hold to a charge, PayPal treats it as a separate transaction. PayPal already placed the hold for $1700 on your account balance, so when the charge came through, they had no choice but to deduct from your bank account. 3 things happened here that probably shouldn't have happened: One, PayPal should just treat the hold and the subsequent charge as one transaction, only deducting $1700 from your account balance once. Two, failing that, PayPal should've declined the card when Apple tried to place the charge since there was insufficent funds in your PayPal account. And three, failing that, your bank should've declined PayPal's request to withdraw $1680 from your bank account if there were insufficent funds. After a few days, the initial hold on the funds will be released and your PayPal account should have a balance of $1700 or somewhere around there for you to deposit into your bank account. Unfortunately, I don't think there's a whole lot Apple will be able to do for you since I highly doubt you were double charged by them.
Like I said before, I personally think PayPal is full of crap by blaming Apple, because I've used my PayPal debit card for purchases elsewhere online and frequently see the double charge until the initial hold clears. Or, I use my PayPal debit card at a gas station, I'll initially see the $1 hold placed when I swiped the card to start the pump, then a few days later I'll see the $1 hold, plus a charge for however much I spent in fuel, and then a few days after that, the $1 hold will disappear, increasing my PayPal balance by a dollar. Contrast that with my Bank of America debit card, where I'll initially get the $1 hold, then a couple days later, that $1 hold will go away and be replaced by a charge for the full amount. So I think it's an issue with PayPal's system.
And just some advice for the future, I always recommend using a credit card for large purchases because you never know when something like this might happen. As long as you pay the entire balance before your credit card bill is due, you won't pay any interest.