Question
I went with friends on an outing. We did not have a refrigerator with us and so the only soda we had with us was warm. I went to a store that sells both cold and warm soda and when the storeowner was preoccupied, I exchanged my warm soda with his cold soda. He charges eight shekels for a bottle of soda and ten shekels for a bottle of cold soda. Do I owe him the two shekels that I saved since my I took cold soda and returned warm soda?
Answer
The first question you should ask is whether you were allowed to act as you did in the first place. There are three places in the gemoro that deal with this issue. In one section of the Gemoro (BM 61B), the Gemoro discusses the pasuk (Vayikro 19, 11) that is the source of the prohibition to steal money, "lo signovu-you shall not steal". The Gemoro questions that the prohibition is superfluous since the prohibition can be derived from alternate sources. The Gemoro answers that the pasuk is written in order to teach that there is a prohibition to steal even if one does not intend to keep the stolen goods. The Gemoro lists two scenarios where a person stole but did not intend to keep what he stole. The first example is one who steals just in order to afflict his victim but intends later to return what he stole. The second example is one who stole in order to repay double since one who steals must return double. Thus, we see even if one only stole in order to do a favor to his victim he still violates the Torah's prohibition to steal.
When the Rambam (Geneivo 1, 2) records this din he writes that the reason for the prohibition in this case is because it will lead the one who stole to steal in other situations as well. Since the Rambam writes a reason for the prohibition, the Lechem Mishna (ibid) writes that the Rambam maintains that the prohibition is only rabbinic. However, the consensus is that the Rambam agrees that the prohibition is from the Torah and only adds a reason for the mitzva. There is very strong support for this approach since the Rambam himself cites this ruling in the Sefer Hamizvos (mitzvos 244) when he records the Biblical prohibition of stealing. Moreover, the Sefer Hachinuch (mitzva 224) explicitly writes the prohibition is Biblical.
Thus, we see that even when one steals in order to benefit his victim by repaying double, he violates the Torah's prohibition to steal. Certainly, you who stole in order to return your theft but not in a manner that would benefit your victim violated the Torah's injunction.
The second place this issue is discussed in the Gemoro (BK 60B) concerns Dovid Hamelech. The Gemoro explains a cryptic remark of the Tanach that when Dovid and his troops were battling the Plishtim they asked the Sanhedrin whether they were allowed to take barley kernels from Jews in order to feed their animals and replace them with more valuable lentils. The Sanhedrin replied that under normal circumstances this would be prohibited but for Dovid who was a king this was permitted. When the Sanhedrin said that normally it was prohibited, they did not cite the pasuk in chumash, lo signovu, but a pasuk in Yechezkeil (chapter 33) that even if one intends to return what he stole, nevertheless, he is called a rosho-a wicked person.
The commentaries question why the Sanhedrin did not cite the pasuk of lo signovu which is in the Torah. The Yad Dovid answers that if we would have only had the pasuk in Chumash we could have understood that the Torah's prohibition applies only in case the victim suffered from the anxiety of thinking that he suffered a loss when he realized his loss and did not know that the theft would be returned. However, Dovid Hamelech's victims, knowing Dovid's high moral standards never had a doubt that Dovid would repay them. Therefore, the Gemoro cites the pasuk in Yechezkeil in order to derive that even if the victim did not suffer any anguish, nevertheless it is forbidden to steal.
The third place this issue is discussed (BB 16A) concerns Job who extolled that one of his virtues was that he would steal fields of orphans and return them after he improved them. The commentaries question Job's proudness since what he did was prohibited. The Yad Rama (199) and Rav Elchonon Wasserman (note 63) answer that the Amoro in the Gemoro who understood Job's words in this manner maintained that Job was not Jewish and for non-Jews theft in this manner is permitted and only Jews are enjoined from acting in this manner.
We should note that the Rosh (BK 6, 12) rules (His opinion is cited by the SA (359, 2).) that if one who stole, immediately replaces the stolen good with a better good he does not violate any prohibition since the victim is happy with the exchange. However, this does not apply to you who did not return anything better than what you took.
From the above, we see that it is Biblically forbidden for a Jew to steal even if he intends to return his theft. Thus, you violated the Biblical injunction against stealing and you are dubbed a rosho by the pasuk.
Turning to your question, whether you presently owe any money it would seem to depend on your intention when you placed your bottle of soda in his refrigerator.
If, at the time when you placed your bottle in his refrigerator, your intention was to cool your bottle of soda and only after it became cool you wanted to return the bottle of soda, you would not owe any money to the storeowner because you stole a bottle of cold soda and returned a bottle of cold soda. Even though you used his refrigerator to cool your bottle of soda you do not owe him money for using his refrigerator to cool your soda in case there was room in his refrigerator to place your bottle inside. Even though you benefitted from the use of his refrigerator since he did not lose anything from your use of his refrigerator, your benefit has the status of ze nehene veze lo choseir -A benefitted from B's possessions but B did not suffer a loss, a benefit for which one is not liable (BK 21A). If the storekeeper does charge for use of his refrigerator you would owe him money for using his refrigerator, but if as usual, he does not charge, your case would be classified ze nehene veze lo choseir just like the Gemoro rules that one who squatted on someone else's vacant property is not obligated to pay rent if the property was not slated for rent, since the owner did not suffer a loss as a result of the squatter's action.
We should note that if you would have otherwise bought a bottle of cold soda from this store (e.g., there are no other similar stores in the vicinity) you would need to pay him for use of his refrigerator since he did suffer a loss of your patronage because you used his refrigerator. The source for this statement is a ruling of the Noda Biyehuda (CM Tinyono 24) in the case where a publisher used the type whose setting was paid for by A to publish a sefer which would rival A's sefer and cause A to earn less money. The Noda Biyehuda ruled that the publisher owes money to A for using the type that he paid for since by publishing a rival sefer he caused A a loss of income. Similarly, if by using the storeowner's refrigerator you caused him to forfeit your patronage you would owe him money for using his refrigerator since it is ze nehene veze choseir.
All the above is only true if you only intended to return the soda only after it cooled off. However, if you intended to pay back your theft by returning your bottle of soda immediately when you placed the soda in his refrigerator you would still owe money to the storekeeper. The reason is because the value of your warm soda is less than the value of the cold soda that you stole and so you still owe the two shekels that are the difference in value.
The Sha'agas Aryeh (81) discusses the Gemoro that we cited above and notes that we see that even though the thief intended at the outset to return his theft, nevertheless he is called a rosho. He explains that even though the prohibition against stealing is a lav henetak le'ase-a negative command which is followed by a positive command to undo the misdeed, that only serves to free him from the punishment of malkus-lashes but it does not remove his status of being a rosho.
In conclusion: What you did was definitely wrong. However, if you gave back the soda with the intention that you only will transfer possession to the store owner after your soda bottle cooled off, you do not owe more money if you would not have otherwise necessarily bought cold soda from this store.
