Before us is a major mishnah: Terumos chapter 6, mishnah 3. The mishnah deals with a person who feeds Terumah to his workers or to his guests - for example, a restaurant owner who serves Terumah to those dining with him or to his employees. The case is not simple at all, since the obligation to repay the principal and the added fifth falls specifically on the one who ate, drank or anointed himself unintentionally; and here the owner is indeed responsible for the loss of the Terumah, yet he himself did not eat any of it.
For comparison: if a person were to take Terumah and pour it down the drain, his status would be that of a robber who must repay the kohen what he took, but he certainly would not pay the added fifth - that payment applies only to eating. Even the principal he would repay only in a monetary sense, as compensation for taking something that was not his, and he would not be required to pay with properly tithed chullin that becomes Terumah. In other words, repayment of monetary value - cash instead of produce - to the kohen who suffered the loss.
But here the Terumah was not poured down the drain; it went down the throats of the workers or the guests. Someone did in fact eat Terumah, but the eating was not his fault - the workers and the guests are not the ones who committed the transgression. The question, then, is how to divide the obligations: who pays for what, who needs atonement, and which payments are actually required.
The view of Rabbi Meir:
"Hu meshalem es hakeren vehem meshalmin es hachomesh" - the one who fed them, the restaurant owner or the employer, repays the kohen the principal component, and his payment is in money. There is no requirement that he pay in food, since he himself did not eat Terumah and it is not for him to restore it. His payment follows the law of a robber, a damager, or one who destroys another's property.
The guests or workers who ate the Terumah, however, need atonement, and therefore they pay the added fifth: if they ate four figs, they must add a fifth fig and give it to the kohen, and that fig must be properly tithed chullin that becomes Terumah. In this way their atonement is achieved and their payment is complete.
It emerges that the kohen who lost his Terumah receives everything due to him: the principal he receives from the employer or the restaurant owner, and the added fifth from those who ate, who need their atonement.
An open question remains here: if there is a principle of preserving the Terumah, as was mentioned in the previous mishnayos, here Terumah has been lost and no one restores it, and nevertheless someone ate Terumah. Some learn that in practice the guests or the workers pay principal and added fifth, only that they sell the principal component back to the kohen and do not absorb the monetary loss, or that they receive reimbursement from the employer or the restaurant owner, and the loss is offset. The mishnah states the bottom line - who pays for what - and the matter remains unresolved.
The view of the Sages:
"Hem meshalmin keren vechomesh, vehu meshalem lahem demei se'udasan" - in the opinion of the Sages, one who ate Terumah unintentionally pays principal and added fifth according to the standard rule, and it makes no difference who is responsible for the mishap. True, the act was done inadvertently, but once it was done, they must pay full compensation and receive complete atonement, like anyone who eats Terumah unintentionally. As for their monetary loss, the employer or the restaurant owner pays them the value of their meal and reimburses them the cost of their food, and so they come out even.
On the face of it, the two views amount to the same thing: in the end the cost of the principal falls on the employer or the restaurant owner - whether through direct payment to the kohen or through reimbursement to his workers; and the cost of the added fifth falls on those who ate the Terumah unintentionally, since they are the ones who need atonement.
The practical difference:
There is a significant difference between them. The going price of produce that is Terumah is lower than the price of ordinary (chullin) produce, and the reason is clear: Terumah may be eaten only in a state of purity, it is liable to become tamei, and once it does it must be burned. Demand for it is therefore lower, and so is its price. From here:
According to Rabbi Meir: The employer or the restaurant owner pays the principal value of the Terumah, according to the rate at which Terumah trades on the market - a relatively low rate.
According to the Sages: Since the employer owed a meal to his workers or to those dining in his restaurant, he must reimburse them for the full cost of a meal - the going price of a meal, or chullin prices, which is what they have coming to them, and not the lower Terumah prices.
In summary: The halachah follows the Sages - those who ate pay the principal and the added fifth, and receive reimbursement according to the higher price, the going price of a meal. They bore the financial loss, but the meal was owed to them, and therefore they receive full reimbursement.