Commentary for Avodah Zarah 68:16

Rava says that he can prove both of these laws from the mishnah in which R. Joshua and R. Ishmael argue as to why cheese made by non-Jews is prohibited. R. Joshua argued that non-Jews curdle their cheese with rennet made from nevelah (an animal not properly slaughtered). To this R. Yishmael responded that the rennet of a burnt offering is permitted. This proves that non-flesh parts of animals from which one may not derive benefit (like the burnt offering) are permitted. Thus dung from an ox sentenced to be stoned is permitted and can be used for betrothal.