If all land is granted in feudal society does that mean that a household knight who is very lucky in the realm of ransoms, say, couldn’t buy himself a small country estate?

Not in the sense of hiring a realtor and going out on the open market, no. 

However, there were ways of parlaying a windfall of cash into acquiring land: 

• you could use the money as a dower for a marriage to a noble family that’s cash-poor but land rich, 
• or you could use the money as a bribe to some lord to grant you one of their manors (which was not uncommon for magnates who owned multiple manors).
• or you could take the money to court in the hopes of buying your way into royal favor and from there into a fiefdom.