Menachos, Chapter Seven, Mishnah 5. This mishnah revolves around the laws of Maaser Sheni, so let us begin with a brief reminder of what it is.
What is Maaser Sheni:
In the seven year Shemittah cycle, the agricultural cycle, there are four years - the first, the second, the fourth and the fifth - in which the farmer, after he has given Terumah Gedolah and Maaser Rishon, separates a tenth of what remains. This produce he must bring up to Jerusalem and eat within its walls. And if the bother of transporting the physical goods themselves is too great, he may transfer the sanctity of the Maaser Sheni food onto coins, go up to Jerusalem with the coins, and spend them there on food to eat.
Perhaps the ideal use of Maaser Sheni money is to purchase Shelamim offerings, which are brought in the Beis HaMikdash and whose meat is eaten during Chol HaMoed - the Shalmei Simchah, sacrificial meat, and nothing is finer than that.
The principle underlying the mishnah:
The Todah offering is really a subcategory of Shelamim, and there is no good reason why a person should not use his money to buy a Todah instead of a Shelamim, if he wishes to bring the loaves along with it and gain the added element of thanksgiving, and eat it in Jerusalem when he goes up there. But there is a separate point, unrelated to this, and it is the main subject of our mishnah: someone who has an obligation to bring an offering may not fund it with Maaser Sheni money; he must spend his own private money - chullin money. Even though there is no inherent obligation to bring a Todah or a Shelamim, once he said "Harei alai todah" and took upon himself to bring it, he turned it into an obligation. And once this obligation rests upon him, he can no longer purchase the Todah with Maaser Sheni money. This is the content of the mishnah.
The first case - "Ha'omer harei alai todah, yavi hi ve'lachmah min ha'chullin":
One who obligates himself to bring a Todah must bring the Todah - the animal itself - and the bread that accompanies it from chullin, meaning not from Maaser Sheni money but out of his own pocket. We said "out of his own pocket" because Maaser Sheni is a sort of forced savings plan that Hashem imposed on a person: to set aside money or food in order to go up to Jerusalem and enjoy himself there. Some would view this as wasted money, but the point is that if he took upon himself an obligation to bring a Todah, he must add additional chullin money to spend in Jerusalem.
The second case - the Todah from chullin and its bread from maaser:
A person says: "Harei alai todah, and I am bringing it from chullin, from my own pocket; but the bread component I wish to bring from the maaser money in my possession." Says the mishnah: he must bring it and its bread from chullin - he must bring both the animal and the accompanying bread from chullin, out of his own pocket, without using Maaser Sheni money. And the obvious question is: why? He accepted upon himself a qualified obligation, and the qualification should hold.
The answer is that a Todah offering has two built in components: the zevach, the animal, and the bread, the loaves. Once he said that he obligates himself to bring a Todah from chullin, at that very moment he obligated himself to bring both the animal and the bread from chullin. And when he afterward tries to qualify the bread portion and say "this will come from maaser," it does not help - it is too late. This is no different from a person who says: "I hereby accept upon myself to be a nazir, but I want to drink wine." Too bad - he is a nazir, and a nazir cannot drink wine, because that is an inseparable part of the package. So too the loaves are an inseparable part of the Todah package, and therefore if he tries to say that the loaves come from his Maaser Sheni money - he cannot.
The third case - the Todah from maaser and its bread from chullin:
Here the person says from the outset: "I take upon myself a todah, and I am purchasing the animal with my ma'aser sheni money, while the loaves that accompany it I will bring from my own pocket" - he brings it, and he is permitted to do as he said. In other words, he never accepted upon himself an independent obligation to fund the purchase of the todah and only afterward tried to use ma'aser sheni money; rather, from the outset he said that his obligation is to use only ma'aser sheni money for the todah, and that obligation takes effect. And since he added on his own initiative an obligation to pay for the bread from his own pocket, he must pay for the loaves from his own pocket, while the animal he may bring from ma'aser sheni money.
The fourth case - the animal and its bread from ma'aser sheni:
If he said: "I am bringing both the todah, the offering, the animal, and also the accompanying bread from my ma'aser sheni money" - he brings it, and he may do so. In other words, his obligation is limited to exactly what he said and no more, and therefore there is no problem with this.
A further qualification - from the money and not from the wheat:
The fourth case does not end there. The Mishnah adds that when a person fulfills his obligation to bring the accompanying bread of the todah with ma'aser sheni, he must do so specifically with ma'aser sheni money. For the ability to bring the loaves from ma'aser sheni stems from the possibility of purchasing the todah itself with ma'aser sheni money, and the two go hand in hand. Therefore he may not take actual ma'aser sheni wheat, which he separated from his field, and prepare from it the loaves that will accompany his todah. Once he has purchased a todah, the loaves must have the same status; and since there is no such thing as a ma'aser sheni animal - for ma'aser sheni applies only to agricultural produce - bringing the animal, the offering, from ma'aser sheni means with ma'aser sheni money. And once his obligation is in terms of money, he must use money and not the wheat itself, both for the animal and for the loaves that accompany it.
In summary: In this Mishnah we learned that one who accepts upon himself an obligation to bring an offering may not fund it with ma'aser sheni money but only from chullin, and that the scope of the obligation is determined by the wording used by the person at the moment he accepted it: one who says "Harei alai todah" without qualification, or "a todah from chullin," has obligated himself in the animal and the loaves from chullin, and a later qualification is of no avail; whereas one who qualifies his words from the outset - his obligation stands as he stated it. And finally we learned that bringing the todah and its loaves from ma'aser sheni means from ma'aser sheni money and not from the wheat itself.