Tevul Yom, Chapter 4, Mishnah 5. As the tractate draws toward its close, the Mishnah turns to rulings that have nothing to do with a tevul yom at all. They are gathered here because of their shared form: each one records a ruling the chachamim originally held, followed by a later, broader position, introduced with the words "barishona hayu omrim" (originally they used to say) and "chazru lomar" (they then said). Our Mishnah presents such a pair concerning the redemption of ma'aser sheni.
Ma'aser sheni is the tithe that must be brought up to Yerushalayim and eaten there in taharah. Alternatively, its holiness may be transferred: it can be redeemed, its sanctity moving onto something else, so that the ma'aser sheni produce itself becomes ordinary food. The question the Mishnah addresses is what may be used for that transfer.
"Barishona hayu omrim, mechalelin al peiros am ha'aretz." At first the ruling was this: a person holding ma'aser sheni could shift its sanctity onto fruit belonging to an am ha'aretz, one who is lax regarding the rules of tumah and taharah and regarding the separation of tithes. In other areas of halachah such produce would not be trusted, yet for this purpose it was deemed suitable to absorb the kedushah being transferred off the ma'aser sheni.
"Chazru lomar, af al me'osav." Later the chachamim widened the leniency further: the redemption may be performed even onto coins belonging to an am ha'aretz. His currency, no less than his produce, can serve as the recipient of the ma'aser sheni's kedushah, and the moment the transfer takes effect the fruit reverts to ordinary status and may be eaten anywhere.
So the Mishnah records a movement in one direction: what began as permission to use an am ha'aretz's produce was later extended to his money as well. The halachah, once established, was applied more widely rather than more narrowly.