We praise not only your willingness to forgo inheritance if halachically required, but also that you are not trying to trick your parents in this regard.
There are times when parents’ gifts are exempt from ma’aser. Rav Moshe Feinstein[1] rules that when parents promise money to their son for his basic needs, the parents have the right to have their son not give ma’aser from their gift, as giving it would in effect compel the parents to give the son more to cover his own needs. Teshuvot V’Hanhagot[2] writes that if that son gives ma’aser against his parents’ conditions, it is considered stealing.
However, this argument does not apply in your case. Your parents are not giving you money for a specific purpose that will not be met if you give ma’aser. Rather, the inheritance will be yours to do as you like, and what you do with it will not impact your parents; , they object only on a “matter of principle.” If indeed you are obligated to give ma’aser on the inheritance, their expression during their lifetime of their strong sentiment against ma’aser is akin to a parent who commands his child to not fulfill a mitzva, which is an illegitimate request.[3]
If your parents are serious about withholding all your inheritance over this matter and this inheritance is a large amount of money, you can be exempted from ma’aser based on the Rama’s ruling[4] that one does not have to spend an exorbitant amount of money on a mitzva. In addition to other mitzvot, this exemption applies to tzedaka, which actually has an upper limit for how much one may give.[5] Of course, tzedaka (ma’aser kesafim falls under its rubric[6]) involves financial loss by its very definition, but here we are talking about a large loss beyond natural tzedaka costs.
In general, there are three opinions regarding the practice of ma’aser kesafim. Some maintain that it is a mitzva from the Torah,[7] some hold it to be a Rabbinic obligation,[8] and yet others maintain that it is simply a proper practice to accept upon oneself.[9]
We believe that the third opinion is the strongest. According to that opinion, one could perform hatarat nedarim on his practice of ma’aser kesafim. We suggest that even if you rely on the above exemption based on the Rama’s ruling that one does not have to spend an exorbitant amount of money on a mitzva, it is good to also do hatarat nedarim in regard to this inheritance.
However, it is better (for your sake and probably also for that of your parents) to obviate the mitzva to begin with, rather than to simply fail to perform it due to substantial loss. Therefore, we would recommend asking your parents to leave you the inheritance in a way through which you are anyway exempt from ma’aser. According to most opinions, one who receives objects or property is not required to give ma’aser based on the objects’ value, unless and until he sells them.[10] According to many opinions, money received that is bindingly earmarked for specific needs is also exempt from ma’aser kesafim.[11] Your part in your parents’ estate can be made to fit into one of these situations. For example, they can create a trust fund for certain purposes (e.g., children’s weddings, education), or you could receive real estate, as opposed to cash. In the case of real estate, if and when you sell the property after quite a while, you will give ma’aser at that point. This would hopefully not contradict your parents’ explicit directives.
There are strong indications that ma’aser kesafim is not an independent mitzva, but is rather a set of rules within tzedaka. Your parents are presumably not against your giving tzedaka, but only annoyed by the level and the automatic nature of giving ma’aser. However you resolve the issue with your parents, you do not need to be a less generous person in the long term. If inheritance, and hopefully other sources and merits, enhance your ability to give tzedaka, at some later point, you might end up giving a similar amount of tzedaka as if you followed the rules of ma’aser kesafim formalistically on the inheritance. (One may give more than 10% when he wants or not rely on leniencies that one used to rely on.) We do not condone actually calculating the amount you are withholding from ma’aser and adding it on later to make up for following your parents’ condition, as this would be dishonest to them. However, it would be acceptable to purposely give more tzedaka in other settings, such that what you give naturally over time will offset what you do not give upon your parents’ passing.