Here is the second mishnah in the first chapter of Maseches Maaser Sheni, which deals with the items that may not be purchased with maaser sheni money: "Ein lokchin avadim u'shefachos u'karka'os u'behemah temei'ah midmei maaser sheni" - a person may not purchase with maaser sheni money:
Male and female slaves.
Land.
A non-kosher animal.
The reason: none of these are fit to be eaten, and being fit to be eaten is one of the conditions required in order to use maaser sheni money for a purchase.
"V'im lav - yochal k'negdan":
If these items were in fact bought with maaser sheni money, the buyer must take food worth the value of that maaser sheni money and eat it in Jerusalem in a state of sanctity, with the sanctity of maaser sheni. This eating functions in a certain sense as a redemption for those items, upon which the sanctity of maaser sheni took hold even though such sanctity is not suited to them.
The commentators explain that in this scenario it would seemingly have been appropriate to say that the transaction is simply invalid. There are two reasons why we do not say so:
We are dealing with a purchase made deliberately, with the knowledge that this was maaser sheni money.
And even if it was done by mistake, the seller may no longer be available, and so there is no way to undo the exchange.
Therefore the mishnah rules "yochal k'negdan" - one must eat food with the sanctity of maaser sheni corresponding to that sum.
Offerings that may not be brought with maaser sheni money:
The mishnah continues: "Ein mevi'in kinei zavim v'kinei zavos v'kinei yoldos, chata'os va'ashamos midmei maaser sheni" - one may not bring the following offerings with maaser sheni money:
The birds brought as the offering of a zav or a zavah.
The birds brought as the offering of a yoledes, a woman who has given birth.
A chatas offering and an asham offering.
And if one did bring them - "yochal k'negdan", he must eat food in Jerusalem with the sanctity of maaser sheni corresponding to that sum.
The halachah rules that even though these are offerings, and even though maaser sheni money is valid for shelamim offerings, it may not be used for these. The reason: all of these are obligatory offerings, which a person is required to bring, whereas the shelamim offerings purchased with maaser sheni money are not obligatory but voluntary. And the halachah is that anything that comes as an obligation, that one bears a responsibility to bring, must come from chullin - from money that is not sanctified. Maaser sheni money is sanctified money, and using it for this purpose is like paying off a debt with maaser sheni money, which is forbidden.
Some explain an additional reason: some of these offerings are olos, and an olah is burned entirely and is not eaten in the way that shelamim are eaten. And even the chatas and the asham, since they are eaten only by kohanim, have a limited circle of those who may eat them; whereas most of the laws of maaser sheni were stated regarding items eaten by the owner, and the owner is not necessarily a kohen.
"Zeh ha'klal":
"Kol shehu chutz la'achilah, li'shesiyah u'lesichah, midmei maaser sheni - yochal k'negdo" - maaser sheni money that was spent on things that are not fit for eating, drinking or anointing (the ordinary use of olive oil) obligates one to eat food with the sanctity of maaser sheni corresponding to it.
This rule includes anything that falls outside the category of what is fit to be eaten, and in addition also things that the Torah specifically excluded from the possibility of being eaten with the sanctity of maaser sheni: water, salt, and produce still attached to the ground.
In summary: in this mishnah we learned which items may not be purchased with maaser sheni money - male and female slaves, land, and a non-kosher animal - because they are not fit to be eaten, and which offerings may not be brought with it - the bird offerings of a zav, a zavah and a yoledes, chata'os and ashamos - because they come as an obligation, and anything that comes as an obligation must come from chullin. And in all these cases, after the fact, "yochal k'negdan". We also examined the rule that any use of maaser sheni money outside of eating, drinking and anointing obligates one to eat food corresponding to it with the sanctity of maaser sheni in Jerusalem, and this includes even water, salt, and produce still attached to the ground.