Meat Cooked in Milk and Flesh Torn in the Field

It was taught: Rabbi Shimon ben Yehuda says in the name of Rabbi Shimon: Meat cooked in milk is forbidden to be eaten but permitted for benefit, as it is said, "For you are a holy people to the LORD your God; you shall not cook a kid in its mother's milk (Deuteronomy 14:21)." And elsewhere it says, "And men of holiness shall you be to Me, and flesh torn in the field you shall not eat (Exodus 22:30)." Just as there it is forbidden to be eaten but permitted for benefit, so too here it is forbidden to be eaten but permitted for benefit. "And flesh torn in the field you shall not eat" - once it has gone outside its boundary, it becomes forbidden. "And flesh torn in the field" - what does the verse teach? Because we find with the second tithe and the firstfruits that even though they went outside their boundary and returned, they are permitted, I might think this too is so; the verse teaches "torn [tereifah]." What is the textual derivation? Rava said: "like a tereifah" - just as a tereifah, once it has been torn, has no further means of becoming permitted, so too the flesh, once it has gone outside its boundary, has no means of becoming permitted.