Entry in the special register shall be an effectual protection of the tenure or farm so registered, unless in a suit instituted by Government in a Civil Court within the period allowed for suits for the recovery of the public revenue a decree be passed pronouncing the registration to have been obtained by fraud, to the injury of the Government revenue. Provided that a tenure or farm in the hands of a bond fide purchaser for value shall not be avoided by reason of such fraud. But the tenure or farm shall be liable to such amount of rent as would have been fair and equitable at the time of the special registry thereof, such amount to be fixed by the Collector.