From wiki: "Knight-service was a form of Feudal land tenure under which a knight held a fief or estate of land termed a knight's fee (fee being synonymous with fief) from an overlord conditional on him as tenant performing military service for his overlord."

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.