Where in the rules does it say that you can choose to set an animal free?
Edit: page 6 under action D, the second paragraph-
During the course of the game, players may rearrange their animals at any time, as long as
these rules are followed. A player may release some or all of their animals at any time, at
will (for example, to make room for other animals in a farmyard space). Animals breed at
the very end of the Harvest (see page 5).