If I want to do a mitzva that is very hard for me, can I say that half the schar should be for me (because I want a yeshua) and half for the ilui neshama/yeshua of someone else?
Thank you for your question.
Essentially, we cannot give the schar for a mitzva to another person. The reason is because the schar that we get for doing a mitzva is not like payment for a job that we worked, and we receive a salary that we can then pass on to another person. Rather the schar that we will get in olam haba is a result of the perfection that our mitzvos have given to our neshoma. Therefore we can’t just give over that perfection to others. There are however a few exceptions to this. One of them is that when one person causes another person to do a mitzva, then he will have a zechus because he caused the mitzva to get done. Therefore, if someone learns extra torah because of someone else’s support, that person has that benefit that he caused the extra torah to be learned.
Regarding your question, if you are doing the mitzva even though it is hard for you, but you gather the strength to do it in order that it should be a zechus for the other person, then they will get the zechus that they caused you do to this difficult mitzva.
Regarding doing mitzvos, and telling Hashem that we want half of the schar to go for a certain yeshua. It is understandable that someone who needs a yeshua will be tempted to do this, however this does not appear to be the correct approach. The first reason is because unless it is regarding giving tzedakah, we don’t make deals with Hashem. If there is a mitzva that we are commanded to do, we have to do it and we can’t tell Hashem, “I’ll do it if you compensate me”. Secondly, there is a rule, “sechar mitzva bhai alma leka” lit. the payment for our mitzvos does not exist in this world. The Chofetz Chaim explains this with a moshul of a young boy that his father died, and didn’t leave him with much, except for two beautiful diamonds. Other than that, the boy didn’t have that much. One day the boy was in the local grocery store and he saw these oversized gumballs, and he really wanted one. He went home and brought the grocer one of the diamonds, and told him, my father left me this rock, can I trade it for some gumballs? The storekeeper absolutely refused. He told the boy that he just can’t do this. After the boy pleaded with him, he explained himself. He said that right now, you really want the gumballs, but one day you will grow up and when you realize that you can me the diamond for some gumballs, you will go to the police, and I will be in big trouble, for shortchanging you in such a severe way. It is the same with us. The sechar that we will get in Olam haba will be far greater than anything we can experience in this world, and it will be a pleasure that will never end. To exchange that for a fleeting pleasure in this world, is underselling the mitzva’s worth. In fact, this world doesn’t have capability to actually pay us for a mitzva, therefore the sechar has to wait until later.
It is or this reason that when we daven we never come to Hashem and ask for a yeshua in our own zechus, rather it is always, pleading with in the zechus of the Avos, or simple because of Hashem’s kindness.
It must be noted that there is an idea of taking on a kabola (bli neder) to do something extra, and that will hopefully serve as a zechus, if Hashem will allow it, however to make an exchange of mitzvos for a yeshua is not done.
May Hashem send you and your friend a yeshua from all of your problems, and a shfe of bracha for you and your family.
Maharam Alshkar 101, Minchas Osher vol. 2 -58.