פירוש על עבודה זרה 102:9

The first mishnah of this chapter taught that one typical way of worshipping the Mercurius idol was to lay stones next to it. In the second mishnah we see ways in which Mercurius was not worshipped. It was not worshipped by having money, clothes or other vessels laid next to it. Since this is not typical Mercurius worship, it is permitted for Jews to use these items.
However, if anything that is normally sacrificed is found next to a Mercurius, these items are forbidden to Jews. Since these items are generally used in idol worship, and evidently also in the worship of Mercurius, they are forbidden if they are found in proximity to the idol.