Avodah Zarah, Chapter One, Mishnah One
1) On the three days preceding the festivals of idolaters, it is forbidden to conduct business with them, to lend articles to them or borrow from them, to lend or borrow any money from them, to repay a debt, or receive repayment from them.
a) Rabbi Judah says: we should receive repayment from them, as this can only depress them;
i) But they [the Rabbis] said to him: even though it is depressing at the time, they are glad of it subsequently.
Exodus 23:13 states, Make no mention of the names of other gods; they shall not be heard on your lips. From the last part of this verse, they shall not be heard on your lips the Rabbis created a midrash that a Jew should avoid giving a non-Jew a reason to bring a sacrifice or libation to his foreign god. Therefore, during the three days preceding pagan holidays, Jews should avoid any business transactions with non-Jews, lest the non-Jew thank his god for this transaction. According to the first opinion in the mishnah, this prohibition works in both directions. It is forbidden for Jews to sell, lend or repay non-Jews and likewise it is forbidden to buy, borrow or receive repayment from them. According to this opinion, all of these transactions may potentially cause the non-Jew to celebrate and therefore should be avoided. Rabbi Judah dissents with regards to receiving repayment from non-Jews. Since repaying a debt causes sorrow to a person, it is permitted to receive repayment during this time, since the non-Jew will not thank his god after having done so. The Rabbis respond to Rabbi Judah that repaying a debt can indeed be a cause of celebration, even if the immediate parting with the money is depressing. Therefore it too is prohibited three days preceding a holiday.
We should note that although this mishnah seems to be of a restrictive nature, it does indeed allow business transactions at any time that is not three days before their festivals. In other words, by forbidding the conduct of business on certain days the mishnah tacitly permits conducting business with non-Jews on other days. This was of course an economic necessity; even before the modern global economy no people could survive without conducting business with other peoples.