Responsa for Bava Batra 252:12

ודלמא בוכרא דאמא הוא גמירי בוכרא דאבא מסי רוקיה בוכרא דאמא לא מסי רוקיה

[the son] did not renounce [his privileges].<span class="x" onmousemove="('comment',' Which the Torah had conferred upon him. Hence the law that the stipulation is null. ');">³³</span> R. Joseph said: [If] one said, 'X is my firstborn son', [the latter] is to receive a double portion.<span class="x" onmousemove="('comment',' His father's word is sufficient in this case to establish his right. ');">³⁴</span> [But if he said]. 'X is a firstborn' [the latter] is not to receive a double portion, for he may have meant,' the firstborn son of his mother'.<span class="x" onmousemove="('comment',' Such a firstborn has to be redeemed from the priest in the same way as the firstborn of a father, but is not entitled to a double portion. ');">³⁵</span>

Teshuvot Maharam