So here is the hypothetical situation:

A vassal knight croaks, and leaves behind an underaged son. The liege lord (a banneret) promptly claims the boy as his ward. However, the Earl informs the liege lord that he still expects to see the full quota of knights at the muster, so the liege lord hires a household knight to defend the manor and to ride off to war when needed.

But now the boy has grown to be a man, eager to reclaim his ancestral estate. This leaves the liege lord one household knight too many to support. What is he to do, if he wants to act honorably?

I would imagine the lord would try to find a suitable lord/bishop/lady for the knight to aid.

Is the lord a pious person concerned for Saxon attacks on churches and monasteries? Perhaps he could send the knight to a nearby house of holiness to have him protect it, and the knight can improve his Latin in the process.

Does he have a friend who could use an extra knight? The lord could send him his way, as best equipped and provisioned as he can, to ease the burden of a new household knight for his friend.

Perhaps he knows a noble lady with lands of her own, and wants to ensure that she is protected until he or someone loyal to him marries her. He might have to come up with a new solution after the nuptials if the husband brings enough household knights of his own, but at least he would take care of the problem for the time being.

Or he could simply ask the knight what he would prefer, though it might ruffle the knight's feathers a bit.