When the next batch of S9's become available and assuming that the price will still be 2100 USD (I am not intending on starting a debate on what the price will be), does Bitmain adjust the BTC amount to reflect the current exchange rate of BTC/USD?
Generally yes. BUT they only do it once per batch from what I've noticed (mind you I dont pay real close attention).
So they set the price when the batch is made available for purchase and the BTC cost stays the same (even if the BTC/USD price changes) until the next.