We are learning Maseches Ma'aser Sheni, chapter 4, mishnah 12. The mishnah deals with a person who told his son where he had placed ma'aser sheni, and afterward something is found that does not match the announcement exactly, whether the location is different or the amount is different. We have three cases before us, and from them we learn when we assume that the ma'aser is what was found, and when we assume that the ma'aser was taken away and the remaining produce is chullin.
The first case - a difference in location:
"Omer livno: ma'aser sheni b'zavis zo, u'matza b'zavis acheres - harei elu chullin" - a father tells his son that he placed ma'aser sheni in a particular corner of the house, and the son finds produce in a different corner, not in the place his father mentioned. This produce is chullin. We do not assume that this is the very produce the father set aside, since he placed it in a specific corner and it is no longer there. Therefore we assume that the ma'aser sheni was removed from its place, and the produce found elsewhere is other produce entirely.
The second case - more than the stated amount is found:
"Hayah sham maneh, u'matza masayim - hashe'ar chullin" - the father told his son that he had placed there the value of a maneh, and in that very corner the son found double that amount, two hundred instead of one hundred. The amount the father announced is ma'aser sheni, and everything beyond it is chullin. In other words, of the two hundred that were found, only a maneh is considered ma'aser sheni.
The third case - less than the stated amount is found:
"Masayim, u'matza maneh - hakol ma'aser" - the father said that he had placed two hundred there, and the son found only a maneh. Even though what was found differs from what the father said, we do not assume that the entire two hundred was taken and that the maneh found is other produce. Rather we say that half of it was taken, and whatever remained in its place, that entire maneh, is ma'aser sheni.
In summary: in this mishnah we learned three cases of a person who told his son where he had placed ma'aser sheni: it was found in a different corner - it is chullin, since we assume the ma'aser was removed and this is other produce; he said a maneh and two hundred were found - the maneh is ma'aser and the rest is chullin; he said two hundred and a maneh was found - it is all ma'aser, since we assume that part of the produce was taken and what remains is the ma'aser itself.