The rule is that a counteroffer terminates the original offer unless (1) the original offeror makes it clear that the offer remains open, or (2) the person making the counteroffer makes it clear that he or she is not rejecting the original offer. See Restatement (Second) Contracts ยง 39(2).
Suppose, for example, that Brian makes his offer to purchase the condo by filling out a standard form agreement for the purchase of real estate. Brianna responds with a counteroffer, which she makes by crossing out Brian's price of $399,000, writing in $451,500 instead.
Brianna's counteroffer does not terminate Brian's offer if...
Brian responds by saying that his offer to purchase at $399,000 is still open.