Niddah, Chapter 4, Mishnah 9. Our mishnah deals with two matters: the law of one who sells himself and his children to a gentile, and the law of one who buys a field from a gentile with regard to bringing bikkurim (first fruits).
"HaMocher Es Atzmo U'Vanav L'Goy - Ein Podin Oso": One who sells himself and his children to a gentile - we do not redeem him.
The Gemara explains that this ruling was not stated in every case, but only regarding someone who has already sold himself twice before:
The first times: we do redeem him.
The third time: we do not redeem him, since he is beyond remedy - we redeem him and he goes right back and sells himself again.
His children: we do redeem them, but only after his death, for as long as he is alive he would go back and sell them again.
One who buys a field from a gentile - the obligation of bikkurim:
The mishnah continues: if a person sold his field to a gentile, and afterwards a Jew bought it from that gentile, the buyer is obligated to bring bikkurim from the fruit, even though it grew while the field was in the gentile's possession. And the reason is: for the betterment of the world.
The Gemara explains that in fact two different enactments were made here, one after the other:
By strict law: one who buys a field from a gentile is obligated to bring bikkurim, since the gentile has no power to remove the obligation of bikkurim. But many people saw in this a proof that the field retained its elevated, holy status, and therefore they reasoned that there was no problem in selling it to a gentile - after all, in the end bikkurim would still be brought from it and it would still remove tumah.
The first enactment: the Sages enacted that one who buys a field from a gentile does not bring bikkurim, so that everyone would know that what grows in a gentile's possession descends in its level of holiness - even though by strict law it is obligated in bikkurim. Out of this sense that the holiness declines, people would refrain from selling their fields to gentiles.
The second enactment: afterwards the Sages saw that there are people who are forced to sell their fields, and once it was enacted that bikkurim are not brought as long as the field is in the gentile's ownership, it turned out that many left their fields in the hands of the gentiles - since in any case, in their view, there was no longer any holiness in them, and they were not careful to redeem them. Therefore the Sages reversed themselves and restored the law to its original state: one who buys a field from a gentile is obligated to bring bikkurim.
In this way the land will not remain in the gentile's hands, but will be returned to holiness - to the holiness of Jewish ownership.
In summary: in this mishnah we learned two laws: one who sells himself and his children to a gentile is not redeemed once he has gone back and sold himself repeatedly, while his children are redeemed after his death; and one who buys a field from a gentile brings bikkurim from it for the betterment of the world - as was explained through the stages of the enactments, which were established so that lands would not be sold to gentiles, and if they were sold - that they should be redeemed and returned to Jewish ownership.