A contract requires a valid offer and valid acceptance.An offer is effective upon receipt.  Although the deadline for the original offer had elapsed, you made a counteroffer and signed it before it expired which constituted a manifestation of present contractual intent.  Acceptance of the counteroffer is effective upon dispatch. Therefore, although the buyer signed it after it had expired, it was effective upon dispatch.  Consequently, you have a binding contract for the sale of your house.