Terumos chapter 6, mishnah 4. The mishnah continues to discuss the consequences of eating Terumah unlawfully, and adds the laws of theft to the obligations of principal and fifth that we have already dealt with.
The laws of theft and double payment:
Regarding one who steals from his fellow the Torah says: "yeshalem shnayim l're'eihu" - "he shall pay double to his fellow." Under certain circumstances the thief must pay twice what he stole, and this is called tashlumei kefel, double payment. From the precise wording of the verse, "to his fellow," we learn to exclude the property of the Beis HaMikdash: one who steals from hekdesh does not pay double.
Instead, there is a separate prohibition in the Torah - me'ilah, the unlawful use of hekdesh property (including stealing it). One who commits me'ilah must pay the principal and a fifth, similar to the laws of Terumah but distinct from them. The mishnah will first discuss one who steals the Terumah of a kohen, and afterwards one who steals Terumah that was consecrated to the Beis HaMikdash.
"HaGonev Terumah v'lo achalah" - one who steals Terumah and does not eat it:
A person who stole Terumah from a kohen and did not eat it - here there are no obligations stemming from eating Terumah unintentionally, since he did not eat any of it, but only ordinary theft. Therefore "meshalem tashlumei kefel, demei terumah" - he pays double, according to the going price of Terumah.
Why is the price of Terumah lower than the price of chullin?
The accepted price of Terumah is lower than the price of that same item as chullin, for reasons of supply and demand:
A limited market: the only people permitted to eat Terumah are kohanim, and anyone who is not a kohen cannot consume it at all.
Even the kohen prefers chullin: while he is tamei he cannot drink his Terumah wine, so it is automatically less attractive in his eyes.
A built-in risk: the Terumah is liable to become tamei, and once it does - even by accident - it is no longer fit to be eaten at all, and it is quite possible that he will never benefit from it.
To illustrate: if the going price of a bottle of chullin wine is ten dollars, a bottle of Terumah wine sells, for the sake of discussion, for only six dollars. The thief therefore does not pay the chullin price but the Terumah price. If the bottle is still in his possession, he returns the bottle of Terumah wine itself and adds a payment of six dollars. And if he no longer has the bottle - it was lost, sold or spilled - he pays double its monetary value: six dollars times two, twelve dollars to the kohen.
"Achalah" - if he ate it:
If he stole Terumah and ate it - two obligations come together here: the obligation of double payment because of the theft, and the obligation of the fifth because of eating Terumah. Moreover, when one eats Terumah there is an obligation to repay with chullin that becomes tithed, that is, to return actual food that becomes Terumah through this repayment, and a monetary payment for the principal is not enough.
Therefore "meshalem shnei keranos va'chomesh" - he pays two principals and a fifth: one principal for the obligation to return the Terumah, a second principal as the double payment, and a fifth because he consumed the Terumah and requires atonement, as the verse states. The one principal and the fifth must come from actual food, while the second principal is paid according to the going prices of Terumah.
In our example: if he drank a bottle of wine containing four cups, and the price of the bottle as ordinary produce is ten dollars while as Terumah it is only six dollars - he must return a full bottle of wine, four cups, as payment of the principal; add a fifth cup as payment of the fifth; and pay an additional six dollars as the double payment, which is monetary in nature, and all of it goes to that same kohen.
Stealing Terumah that was dedicated to the Beis HaMikdash:
Until now we have been dealing with Terumah stolen from a private individual. How could Terumah become the property of the Beis HaMikdash? A kohen who had Terumah in his possession and wished to give it to the Beis HaMikdash as a donation for the upkeep of the Temple - not as material for an offering, but so that the Beis HaMikdash would sell it and use the proceeds to fund the needs of the Temple - the result is that what had been Terumah in his private possession has become hekdesh.
One who steals from the Beis HaMikdash is not liable for double payment, but one who misuses Temple property is liable to pay a fifth - a fifth that is distinct from the fifth a person pays for eating Terumah. Therefore one who steals Terumah belonging to hekdesh and eats it is liable for two fifths: one for eating the Terumah, and one for me'ilah, the unlawful use of the property of the Beis HaMikdash (all of this assuming it was done unintentionally).
This is what the Mishnah means when it says: "meshalem shnei chumashin v'keren, she'ein b'hekdesh tashlumei kefel" - two payments of a fifth, one for me'ilah and one for eating Terumah unintentionally, and one payment of principal. The principal and the fifth that stem from the Terumah come from ordinary produce, since he must return food that is subject to tithing, while the fifth that stems from me'ilah may be paid in cash, provided that it is calculated according to the value of the hekdesh, meaning according to Terumah prices and not according to the prices of ordinary produce. And there is no double payment here, for the verse states "he shall pay double to his fellow" - to his fellow, and not to hekdesh.
In our example: he drank a bottle of Terumah wine worth six dollars that a kohen had given to the Beis HaMikdash - he returns a bottle of wine as the principal, adds another cup of wine as the first fifth, and adds another cup of wine or its monetary value as the second fifth, since the fifth of me'ilah may be paid in cash: if the bottle is worth six dollars, its fifth is a dollar fifty, and that is what he gives to the Beis HaMikdash.
In summary: in this Mishnah we learned three cases. One who steals Terumah and does not eat it pays double according to the value of Terumah, which is lower than the value of ordinary produce because of the limited demand and the risk of tumah. If he ate it, he pays two principals and a fifth: a principal and a fifth from ordinary produce that is subject to tithing, and a second principal as the double payment according to the value of Terumah. If he stole Terumah belonging to hekdesh and ate it, he pays two fifths, one for eating Terumah and one for me'ilah, plus a principal, and he does not pay double, since there is no double payment for hekdesh.