When I have money that I don’t need for a while, I deposit it with a gemach. About a year ago, I deposited twenty-five thousand dollars, twenty of which was mine and five were my ma’aseir money. At various times I withdrew money. Recently, I withdrew money and, according to my records, I no longer had any money left in the gemach. However, the manager of the gemach said that according to his records I have a balance of three thousand dollars. I should note that the manager is very careful. He writes down every transaction immediately, whereas I record transactions when I remember to do so, which can sometimes be a day or two later. I have two questions: May I accept the three thousand dollars since, according to my records, my balance is zero? If I may accept the money, may I use it for personal needs or must I consider it ma’aseir?
Since you don’t record your transactions immediately and the manager of the gemach does, you may rely on his records and accept the money. The source is the Shach (91, 25) who cites a Rosh that when one records his transactions immediately he may even swear based on what is recorded in his ledger since it is reliable. However, records that are not recorded immediately (like yours) are considered legally to be unreliable. Therefore, since the gemach’s records are reliable and yours are not, you may rely on the manager of the gemach and accept the money.
Since we have established that you may accept the money, we must consider your second question i.e. whether the money is yours or it belongs to ma’aseir. If we were dealing with ordinary money and the question was only whether the money is yours or it belongs to another person who left money with you, then you may consider the money as yours. This is similar to the Gemara (Bechoros 18B) which states that if a person asked someone to watch his lamb and that person kept the lamb together with his own lambs and one lamb died, he can assume that the lamb that died belonged to the other party. The reason is because of the rule hamotsei meichaveiro olov horayo: one who wants to retrieve something from someone else must prove his claim. Since the money is by you or by someone to whom you have entrusted it, and it is not certain that the money belongs to someone else, you may keep the money.
However, since in your case the “other party” is ma’aseir money, we must discuss the laws of ma’aseir. Normally, when one sets aside ma’aseir money he is effectively setting aside the money to be used exclusively for paying for mitzvos for which he was not previously liable. The reason is because that is what people generally do with their ma’aseir money. Therefore, we say that that was the meaning of his words or actions.
What is important for us is that by doing so, one is effectively pronouncing a neder to use the money for this purpose. The source for this is two statements of the Gemoro. In the case of money that is set aside to be used for poor people, the source is a Gemoro (Rosh Hashana 6A) that derives from the word “beficho” that one who says he will give money to the poor is pronouncing a neder, a vow. Even if ma’aseir money may be used for other mitzvah purposes, many maintain that it is a vow since the Gemoro (Nedarim 8A) states that one who says, “I will get up in the morning and study this chapter (of Torah),” has pronounced a vow to carry out what he said. The Ran (commentary thereon) explains that all mitzvos are equivalent to tsedokoh and the word beficho refers to them as well. This is also the opinion of the Rambam (Negative Precepts 157). The Radvaz (2, 698) disagrees and maintains that it is not a full-fledged vow. However, the Chazon Ish (Yoreh Deah 153, 5) indicates that one should follow the stringent opinion.
Therefore, if one sets aside money for use as tsedokoh he has pronounced a vow and if he does not use it as tsedokoh but for his own personal use he will violate two negative commandments of the Torah: lo yacheil, do not profane your speech, and lo se’acheir, do not tarry to fulfill your vows.
We find in the Gemara (Chulin 134A) that if one has a doubt that pertains to issurim he must be stringent. Thus, in your situation since, if you will use money that was designated as ma’aseir for your own use and not for mitzvah use, you will violate two Biblical laws, you must be stringent and treat the money as if it were full-fledged ma’aseir money and use it for mitzvah purposes.
Normally, the halacha (Rambam, Nedarim 13, 25) is that one is not supposed to undo nedarim for mitzvah use by means of hatoras nedarim since it is a mitzvah to carry out one’s vow. Moreover, the Radvaz (4, 134) rules that one who frees a person from fulfilling a vow to do a mitzvo is excommunicated. However, in your situation, it would seem that you may have your vow annulled since you never would have made a vow if you had known that the money would be mixed together with your personal money.
The Chazon Ish (cited in Derech Emuno Matnas Aniyim 8, 8) advised people to state that any money which they set aside for ma’aseir should not become tsedokoh money until the time it is given to a poor person or to the gabbai of a tsedokoh. The reason is that if one sets it aside for tsedokoh use he must give it immediately if a worthy cause presents itself because the pasuk (Devorim 23, 22) writes that one who tarries to fulfill his vow violates a negative command of the Torah. If you did that, then you would have no problem keeping the money for yourself because the issue is purely monetary and not issur, and as we wrote above the rule is hamotsei mechaveiro olov horayo, so that if one is in doubt he may keep the money for his own use.
It should be noted further that while it is a big mitzvah to give ma’aseir one should perform the mitzvah without involving himself with vows. Therefore, the Chafetz Chaim (Ahavas Chessed 2, 18) writes that one should state at the outset that he is accepting upon himself to always give ma’aseir bli neder–without it becoming a neder.
In conclusion: If you followed the Chazon Ish’s advice then you may keep the money for yourself. However, if you did not, you must first perform hatoras nedarim and then you may use the money for personal needs.