מדרש על שביעית 8:3

3) (Vayikra 25:12) ("For it is Yovel; holy shall it be to you. From the field shall you eat its produce") "For it is Yovel; holy": Just as (the holiness of) what is holy attaches itself to its money (i.e., the money with which it was redeemed), so, (the prohibition of) shevi'ith attaches itself to its money. But (in that case, let us say that) just as with what is holy the money is attached but the object loses its holiness, so, with shevi'ith (i.e., let the fruit itself be permitted)! It is, therefore, written "shall it be" — it (Yovel) retains its sanctity. So that "shevi'ith" attaches to the very last (object) and the (original) fruit is forbidden. How so? If he took meat in exchange for the fruits of shevi'ith, both are removed on shevi'ith. If he exchanged the meat for fish, the meat leaves (the category of "shevi'ith") and the fish enters. (If he exchanged) the fish for oil, the fish leaves and the oil is attached. The very last is attached and the (original) fruit remains forbidden.