There are different sorts of cases that are included in the broad prohibition of geneivat da’at (deceiving someone). One case is when one sells a defective item without the buyer knowing about the defect.[1] Such a sale is forbidden even when the buyer pays a price that is reasonable given the object’s actual value.
The same gemara includes several cases in which the deceived party loses nothing on any level, but the deception is nevertheless forbidden. In one such case, Reuven makes Shimon think that Reuven intends to give him something, when in fact Reuven does not have that intention. Another case is when Reuven urges Shimon to eat with him when he knows Shimon will not take him up on the offer. Yet another is when one brings to his friend a utensil in a way that looks like its content is of significant value, when in truth it is not.
Furthermore, the gemara forbids even cases in which one is indeed giving something of significant value, if the recipient will view the act as more generous than it actually is. Thus, one may not open a closed barrel of wine before a guest if most of it is earmarked for imminent sale. (Since wine of newly opened barrels tastes better than that of barrels that have been open for a while, opening a new barrel for the guest appears to be a special gesture to him.) Rather, says the gemara, one must inform the guest that he would have had to open the barrel soon anyway.
Based on the broad possibilities for a prohibition of making others more grateful to you than they should be, it would appear, at first glance, that it is halachically problematic to lead your in-laws to believe you bought them a present when you actually bought it for yourself. Nonetheless, it seems that for one or more of the following reasons, you are not required to inform them of their mistake.
First, we must examine the reasoning behind the prohibition of this type of geneivat da’at. Rashi[2] explains that the giver causes the recipient to feel that he owes the giver more reciprocity than he actually does. If the recipient of the favor/gesture were to realize the true situation, he would be less likely to invest in generosity in return. Thus, if there is no reason to expect any change in reciprocity due to the process of giving, it is likely permitted to present a more positive picture than exists. In your case, we would consider that parents (in-law) usually give their children gifts in a manner that is unrelated to relatively small gifts that their children give them. (Although we do not ordinarily make such distinctions based on the spirit of a law regarding prohibitions, a prohibition like this one is totally situational by its very nature, thus making it likely that this distinction will indeed affect the halacha.)
The following story recounted in that gemara[3] is also instructive. Two rabbis were traveling when they chanced upon a third rabbi. When they met, the third rabbi expressed his appreciation that they had come to greet him. One of the first two politely corrected the third rabbi’s mistake to avoid deceiving him. The second rabbi told the first one that he was mistaken in disappointing the third rabbi, claiming that there was no problem of geneivat da’at because the third rabbi had “deceived himself.” The Shulchan Aruch[4] rules like the second rabbi; if the “recipient” should have realized that he actually did not receive a favor, the “giver” does not have to correct him. We can derive a leniency and perhaps also a stringency from this ruling. The stringency is that a giver might need to correct a misimpression, even if he did not act purposely to create the perception, if it turns out that he had unwittingly created the misperception. The leniency is that if the unintended recipient should not have jumped to the erroneous conclusion, the “giver” does not have to correct it. You would know best whether this idea applies to your case – that is, whether your in-laws should have assumed that you bought it for yourself rather than for them.
Another leniency is that it is permitted to give the wrong impression if the motivation of the “deception” is not to win favor but for the honor of the recipient – for example, if as a result, other people will think that the recipient is a person who is deserving of significant honor.[5] If we apply the concept of honor here, it makes sense to be concerned that it might be embarrassing to your in-laws for them to be told that they made a mistake in assuming it was a present for them, and it seems that you would be justified in going along with that assumption. On the other hand, it is possible that they would not be embarrassed, as it was your mistake in not telling them that the item was coming for you.
Another reason there may not be a problem of geneivat da’at here is that by letting them keep the item, you actually are giving them a present now. Thus, it turns out that they really do have reason to be grateful for your generosity and your care about their happiness. This is the case even if the background of what was nice on your part is different from the way they perceived it. When one gives an actual present whose level of generosity is equivalent to what the recipient understood (as opposed to the case of opening the barrel of wine), we do not find an obligation to divulge all the circumstances under which it was given. For example, if you gave a nice present thanks to your sister-in-law’s suggestion, you are not required to say that the idea came from her. Similarly, in your case, you do not have to mention that the idea for the present came from your in-laws’ mistake.