Commentary for Avodah Zarah 64:10

The mishnah taught that it is prohibited to derive benefit from skins pierced at the heart. It is assumed that such skins were part of idolatrous practice. This section defines precisely what this heart-pierced skin is.
The presence of the blood is necessary to determine whether the hole was made while the animal was alive. If the hole was made after its death, then there should be no blood and it is permitted. Unless the skin was salted in which case we can assume that the salt removed the blood.

All of the amoraim agree that the halakhah follows Rabban Shimon ben Gamaliel—only a round tear is prohibited.
The dispute between Abaye and R. Joseph is over whether it is important to know whether or not it is important to know if there was a dispute between Rabban Shimon ben Gamaliel and the other sages.

The mishnah allows a Jew to derive benefit from meat that an idolater was bringing to his temple. The Talmud notes that this does not accord with R. Eliezer for R. Eliezer thinks that in general an idolater has idolatry on his mind and thus will already have dedicated this meat to his god, even before it arrives at the Temple. By implication, the mishnah would hold that the idolater has not yet made up his mind whether this meat will be a sacrifice or not.

The assumption of the mishnah is that any meat that came out of a place of idolatry must have been used in idolatrous worship.
The mishnah says that it is prohibited to derive benefit from meat that was used in idolatry. In a roundabout way, this rule is connected with the opinion of R. Judah ben Batera. R. Judah ben Batera said that we learn the rule that products of idolatrous sacrifice defile by overhanging (this means that if they are over an object, the object underneath is ritually defiled) by the comparison with the dead body. Rashi points out that in this context there is another level to the comparison—just as it is prohibited to derive benefit from the dead body, so too it is prohibited to derive benefit from products of idolatry.

Shmuel’s explanation for the mishnah is in line with what we learned way back in chapter one. It is prohibited to do anything that causes an idolater to go and thank his gods. Thus it is forbidden to engage in business with him on his way to an idolatrous place. But on his way back, it is permitted.

The opposite is true with an Israelite. On his way there, one can engage in business with him because we can always hope that he will change his mind and turn away from the festival. But once attracted to idolatry, he will not be able to break away from it and therefore it is forbidden to engage in business with him.