מדרש אויף בראשית 17:27

Rabbi Jochanan said: All heathens who come to Israel are circumcised by their own freewill and with their consent, and in the fear of Heaven are they circumcised. We do not believe a proselyte until seven generations (have passed), so that the waters should not return to their source. But slaves are circumcised both by their freewill and with their consent as well as without their consent, and no confidence is placed in slaves. Likewise with all the slaves who were circumcised with our father Abraham, they did not remain true (converts) in Israel, neither they nor their seed, because it is said, "All the men of his house, those born in the house, and those bought with money of the stranger, were circumcised with him" (Gen. 17:27). Why did he circumcise them? Because of purity, so that they should not defile their masters with their food and with their drink, for whosoever eateth with an uncircumcised person is as though he were eating flesh of abomination. All who bathe with the uncircumcised are as though they bathed with carrion, and all who touch an uncircumcised person are as though they touched the dead, for in their lifetime they are like (the) dead; and in their death they are like the carrion of the beast, and their prayer does not come before the Holy One, blessed be He, as it is said, "The dead praise not the Lord" (Ps. 115:17). But Israel who are circumcised, their prayer comes before the Holy One, blessed be He, || like a sweet savour, as it is said, "But we will bless the Lord from this time forth and for evermore. Praise ye the Lord" (Ps. 115:18).

“And all the men of his household, born in the house or purchased with money from a foreigner, were circumcised with him” (Genesis 17:27).
“And all the men of his household, born in the house or purchased with money” – it is taught: One may go to an idolatrous fair,³⁷A fair where some of the proceedings go towards idol worship. [even] during the intermediate days of the Festival, to purchase from them houses, fields, vineyards,³⁸Although attending idolatrous fairs is usually forbidden, the transfer of property in the land of Israel from idolaters to Jews is important enough to outweigh the prohibition. slaves and maidservants. Rabbi Ami said in the name of Reish Lakish; It is not only for circumcised slaves³⁹That is, slaves that had been owned by Jews and converted to Judaism. It is certainly of great urgency to redeem such a slave from the possession of idolaters, under whose mastery he would not be able to practice his religion. that it is permitted for him, but even for uncircumcised ones, because he brings them under the wings of the Divine Presence.⁴⁰Even if they had not been converted beforehand, the Jewish buyer would have an opportunity to do so now.
Rabbi Yehoshua ben Levi asked before Reish Lakish, saying to him: ‘What is the ruling regarding the purchase of slaves from idolaters?’ He said to him: ‘Regarding when are you asking me, on a Festival? It is taught: Even on Shabbat. Likewise, if one purchases a courtyard⁴¹From an idolater. in the Land of Israel [on Shabbat], he says to him: I offer such and such a price, to be paid tomorrow.’⁴²Although the deal may be completed on Shabbat, he may not pay the money on Shabbat. It is because the Land of Israel is so beloved that it is permitted for him to say so.
Ḥizkiya taught: “[…to build siegeworks against the city that makes war with you,] until it is subdued” (Deuteronomy 20:20) – even on Shabbat,⁴³One may wage war to conquer a city in the Land of Israel even on Shabbat. as we find that Jericho was conquered only on Shabbat.⁴⁴It was conquered on “the seventh day” (Joshua 6:15).
There are three [idolatrous] fairs: The Gaza fair, the Akko fair, and the Botnan fair, and the most obvious one of all is the Botnan fair.⁴⁵It is obvious that it is dedicated to idol worship.
Abraham said: ‘Before I was circumcised, passersby would come in to me. Perhaps now that I am circumcised they will no longer come in to me.’ The Holy One blessed be He said to him: ‘Before you were circumcised, people would come to you. Now, I, in My glory, am coming and revealing Myself to you.’ That is what is written: “The Lord appeared to him in the plains of Mamre” (Genesis 18:1).