I think I've finally worked out what is going on in Ruth 4. I'll make a note here as a place where I can come and find this again when I need it. Do comment below if I've missed something.
There are 3 Old Testament laws in play here.
Leviticus 25:23-28 says that, because all the land is really God's, should someone sell part of their land to alleviate their poverty, the buyer cannot regard it as theirs absolutely. A relative of the person they bought it from must be allowed to redeem, or buy back, that land, and the price for that is to be calculated fairly.