(Mishnah): If a person says, "My firstborn son ploni will not receive a double portion'' or "My son ploni will not inherit with his brothers'', this does not take effect, as he made a stipulation against the Torah.

If a person (who is deathly ill) divides his property verbally, if he gave more to one son than to another, or he gave the firstborn the same as everyone else, his words take effect.

If he said that this is their inheritance, it does not take effect.

If he wrote at the beginning, the middle or the end that it is a gift, it takes effect.

(Gemara): R. Ila appeared to once give a firstborn a portion equivalent to the other brothers.

R. Chagai to R. Ila: Isn't there an explicit pasuk that states (Devarim 21:16), "he will not be able to give the son of the beloved (wife) precedence of birthright''?

R. Elazar: I affirm that he is (technically) able to do so, but he is not permitted to do it.

R. Chagai: If so, R. Ila must have allowed him to give it as a gift rather than an inheritance.