One who divorces his wife

because he believes her to be barren

Rabbi Yehuda says

he may not remarry her

and the Sages say

he may remarry her.

If she marries another

and has children (thus showing that she is not barren)

and she now demands her kesuba (which was previously denied to her on the basis of her having been barren)

Rabbi Yehuda says

we say to her

your silence

is better than your words (as this argument may lead to the divorce being voided as it was based on her ex-husband's erroneous assumption that he would not have to pay the kesuba).

One who [on multiple occassions] sells himself

and his children

to a gentile

we do not redeem him

but we do redeem his children

after the death of their father (to prevent their assimilating).

One who sells his field to a gentile

must purchase and bring from it

bikkurim

for the public good (i.e. to discourage selling the land of Eretz Yisroel to non-Jews).

One who sells his field to a gentile

and a Jew buys it back from him

the purchaser must bring bikkurim from it (even on the fruit that had ripened while under the ownership of the non-Jew

for the public good (by showing that the land retains its kedusha even while in the posession of non-Jews we hope to encourage Jews to buy it back).