A lord dont sell a fief like this. They was not an economic society but a feodal one.
So, why they should give a land ?
- If they are totally broken. They could try to sell for money, but this should be rare. And they cannot sell to an outsider lord. You must be under the same ruler (or the ruler must be the seller and be broken).
- If they have too many castles, they can try to exchange it for a new and rich vassal. So, you can propose a multi-fiefed lord to take one if you give them money and follow him (right now, we have king on top, and under vassal or mercenary. Maybe we can have more layers with dukes and under some counts ?).
- If you have the same or upper level of nobility and you cannot become a vassal, you should be able to buy for denars, but you should also give something else. Influence, alliance or peace deal. Denar are important, but in a feodal society, honnor, land and power was way more important.