“ It does not appear that the proposition has been accepted; and if not, the amount in controversy remains unaffected. But had the alleged reduction been made by an actual payment, the jurisdiction of the court would not be taken away. The jurisdictional facts existed at the time of issuing and serving the writ of error. By its issue and service the court obtained jurisdiction over the cause, and this jurisdiction once acquired, cannot be taken away by any change in the value of the subject of controversy. ”