Except as provided by subsection (b) of this section, if a judgment of condemnation is executed against a garnishee or the garnishee pays the judgment, he may plead the execution or payment in bar in an action brought against him by the defendant even though the judgment is later reversed or set aside. The garnishee may not plead the execution or payment if at the time of execution or payment the judgment or its execution is stayed by order of court.