The Duchy of Sagan (in Polish, księstwo żagańskie) was not sovereign in any way and it was rather a hereditary estate. Those who possesed Sagan were called Dukes of Sagan because historically they were successors of the Dukes of Silesia from the Polish House of Piast, who ruled the region divided in many small duchies until the 16th or 17th century. One of the historical Silesian duchies was Sagan and after its last ruler died and his line totally died out, it passed to the Bohemian Crown, of which the former Dukes were vassals. So, the so-called Duchy fell to the Crown and it became a semi-feudal property which can be given and sold. Those who held Sagan used the title of Dukes of that place but it was just a noble title of the Kingdom. And because the Courland princesses were not successors of the Piast Dukes of Sagan, they were just heirs to their father's property. Thus, it could pass to them.