On Daf 28b the Mishna says "re'eh heich teshivenu lo" meaning that, as the Gemara explains, shelo ya'achil egel la'agalim, etc. This means one should not spend the value of an animal to support it. Rashi in the Mishna, however, says one should not spend **half** the value of an animal to support it. Why does Rashi apparently change the Gemara in this way?
The Kos Yeshu'os addresses your point. He explains that when the Gemara says "she'Lo Ya'achil Egel la'Agalim," Rashi understands the intention to be that he must not feed an Egel to *two* Agalim (since "Agalim" is in the plural). That is, if a person finds two Agalim, he must not feed them *half* of their total value, as Rashi writes in DH she'Lo Ya'achid. (Some of the manuscripts of Rashi indeed have this Girsa in Rashi, "Im Shenayim Agalim Matza....")