Feudal serfs were bound to the 'land' in a sort of debtor-lendor scenario, and could be sold along with the land beneath their feet. They just couldnt' be sold as individuals.
However, knights, squires, barons, etc. were bound to serve their superiors by oath. Its the oath that kept the political and military structure together.
There still was debt to be paid even by knights. Every year the debt obligation had to be met via service or hard ca$h.
EDIT: changing "lords" by knights in the Middle Ages was common. I.e. for few years a knight served a French king just to serve German Prince for a few years after that. So, it looks like they couldn't have taken their oaths too seriously or there had to be an easy way to have the whole thing cancelled.
Don't remember the exact date or edict name, but when in the Roman Empire a law was introduced banning debtors from moving too far from their creditors - that's when according to many historians Middle Ages have started.