There are many situations where one is in the midst of fulfilling one mitzvah, when another mitzvah then presents itself. For example, in the middle of prayer, someone asks for charity. Does one have to fulfill the second mitzvah, or is he exempt on account of being involved in the fulfillment of the first? Thanks for your very clear and detailed answers.
There is indeed a general principle that one’s being engaged in the performance of a mitzvah exempts one from needing to fulfill another mitzvah that comes his way.
The reasoning behind this is as follows: If one who is idle shuns the performance of a mitzvah it would be tantamount to denying
However, based on this reasoning, there are several exceptions and qualifications.
If the mitzvah that he’s currently engaged in can be done at a later time without diminishing it, and the other mitzvah that presents itself can only be done now, and afterward it will no longer be applicable, one should temporarily postpone his observance of the first in order to fulfill the other. In this way he fulfills both mitzvot as opposed to just one. And refraining from fulfilling the second mitzvah in this case is somewhat like disregarding the mitzvot, since doing so would be choosing to do one mitzvah when he could have done two.
Similarly, even if both mitzvot are time-limited and must be done now or never, but the person can actually do both simultaneously, he must not shun the later mitzvah because of his previous involvement in the former. And this is because his engagement in the first mitzvah is not so engaging as to preclude his ability to concurrently perform the other. The example you give would fit this category since most people in most parts of prayer can give charity during prayer without distracting their intention. Exceptions to this would be during the first part of Shema or during the Amida. Since here one needs utmost intention, it would be forbidden to interrupt for charity.
Thus, only in a situation where one could not perform both mitzvot, neither at different times nor at the same time, would one’s involvement in the one exempt him from the other. And in most cases, the mitzvah that he is already engaged in takes precedence over the other, even if the other is a “greater” mitzvah (except in a case of monetary loss or physical danger). But if one has not begun the performance of either, and both are presented simultaneously, the more “important” mitzvah takes precedence. An example would be when a Torah mitzvah conflicts with a Rabbinic mitzvah, and only one of them can be fulfilled to the exclusion of the other, the Torah mitzvah takes precedence.
One notable exception to this is the otherwise all-important mitzvah of Torah study, where even if one is currently engaged in this prime mitzvah he must suspend his learning in order to fulfill even the “least significant” of mitzvot.
The reason for this is that since the a major purpose of Torah study is to learn how to perform
That being said, there is an important qualification to this exception of suspending Torah study in order to fulfill another mitzvah which will pass if he continues to learn without fulfilling it. And that is when the mitzvah can be done only by him and no one else. For example, regarding his own mitzvah of prayer or tefillin which are limited in time and are incumbent solely on him. But if the mitzvah can be performed equally well by someone else who is not currently studying, here the primacy of Torah study over other mitzvot takes precedence, and he must continue studying and leave the performance of the mitzvah to others.