The gemara on page 101 there is no argument. One is talking about a field that usually planted and the other is a field that is not usually planted.
Though the simple way to understand this is that Rav that say the buyer gets back both the קרן ושבח that means for a field that is usually planted.
However it could be that Tosphot mean to take this even further. That is he might mean that Rav and Shmuel do not disagree even about the fundamental law. They both agree when the buyer is the one that did the work and the improvements , then the owner pays him. And that is the law of Rav. And when the thief did the improvements the owner pays the thief and not the buyer. The buyer in any case is getting back what he paid for the property because the improvements the thief did are included in the price that he paid for the field and that he gets back from the thief.
But who did the work on the field can not be the difference between the higher or lower price on page 101 (ידו על העליונה) That was a thought I had that I put into the my little book on Shas but I realized while sitting here in the hospital that that can not be right because the gemara itself states the difference is only dependent on one thing,-if the field is usually planted or not.