Question
I am an avreich who is supported by my irreligious father. My grandfather was very religious but my father "left the path." Since my twin sons are going to celebrate their bar mitzva in a year, I asked my father for money to buy them tefillin. My father said that I needn't to buy tefillin because he has two pairs of old tefillin in his attic which aren't being used. I took the tefillin to be checked figuring the tefillin would be posul. The sofer said the tefillin are beautiful and the parshiyos were written by the famous Nachman Sofer Papirna and the batim were made by the famous Nesaneil Tefilinsky and are worth over a hundred thousand dollars! My father is not wealthy and I am virtually certain that if I report the truth to him, he would want the tefillin back to sell them and give me the money to buy ordinary good quality tefillin. Am I obligated to report the facts to my father.
Answer
The issue of the relationship one has with something valuable whose value is not known by its owner or the one who affected the object comes up in many areas of Choshen Mishpot.
One case that is discussed in the Gemara (BK 62A) concerns the liability of a person who kicked someone else's box into the water not knowing that inside the box were precious stones. The Rambam, whose position is recorded in SA (388, 1), understands that the Gemara was unable to decide whether the one who kicked the box is liable for the precious stones.
Another case that is discussed in the Mordechai (6, 207) is where A borrowed a sword, which belonged to a gentile, from B (who had permission to lend the sword), and the sword was lost by A. When the gentile came to B for the return of his sword, it turned out that the sword was very valuable. The question that was asked to the Mordechai is whether A is liable for the full value of the sword. A argued that he was unaware that the sword was so valuable, and he never intended to accept responsibility for such a valuable sword. The Mordechai tended to agree with A's argument. The Shach (72, 40) also tends to side that A is not liable because of his contention that he never intended to accept responsibility for a precious sword.
We see from these cases that not knowing the true value of an object is considered a lack of da'as-knowledge, which is necessary to create liability. Similarly, da'as is critical in order to transfer ownership. Therefore, in your situation as your father was unaware that the tefillin were so valuable, he did not have da'as to give you these tefillin since he was unaware that the tefillin are so valuable. Since da'as is critical in order to affect a transfer of ownership the present is not effective and the tefillin still belong to your father.
Moreover, in your situation, your assessment is that your father never would have given you the tefillin at all, which is a much greater lack of da'as than the case of the Mordechai where the person accepted responsibility for the sword but just not for the additional value.
Also, since your father gave you a present the transaction is much easier to void. The source for this is a ruling of the Ramo (207, 4), based on the Shiltei Gibborim (Kesubos 56A, 8). The Ramo rules that a sale can only be canceled if one stipulated a condition that was not fulfilled by the other party or, at the very minimum (as we saw in the previous article), one has to have made his intentions known to the other party. However, in order to void a present, it suffices if the person who gave the present did not intend in his heart to give the present. This means is that even though normally the rule is that devorim shebalev einom devorim, with regard to a present devorim shebalev are devorim.
The proof that the Shiltei Gibborim brings is from the Gemara that rules that if someone who was on his deathbed distributed his assets, the presents are nullified if the sick person recovers from his illness since we assume that the only reason the sick person distributed his assets is because he thought he would succumb to his illness even though the sick person did not state any conditions.
The Sema (207, 10) explains that the rationale is that people do not give presents in general. Therefore, we tend to nullify presents.
This is certainly true in your case since according to your assessment your father did not intend in his heart to give you such valuable tefillin. Therefore, since we rule that what one thinks is effective, the present your father gave you is void.
However, there is a machlokes (OC 153, 10) whether your father is allowed to sell the tefillin since many, including the Rambam, rule that one is only allowed to sell tefillin if he does not have enough money to marry off his children or to learn Torah. Therefore, perhaps you can tell your father that the tefillin are valuable and ask him if he still wants to give them to his grandchildren, or perhaps he would rather keep them for himself since one set is his bar mitzva tefillin.
In conclusion: If you are convinced that your father would not want to give the tefillin to your sons if he knew they are so valuable, you must inform your father and give them back to him if he wants them.
