Commentary for Kiddushin 38:14
אמר חזקיה שאני הכא דאמר קרא (שמות כא, ז) לאמה פעמים שאינו מוכרה אלא לאמה בלבד
this is R'Meir's opinion. But the Sages maintain: If he wishes to designate her, he can do so, because he [her father] has stipulated contrary to what is written in the Torah, and he who makes a stipulation contrary to what is decreed in the Torah, his stipulation is null.
Daf Shevui to Kiddushin
Hizkiyah explains that the case of the slave is different because the Torah explicitly says that the father can sell her just to be a slave. In general, though, R. Meir would agree that one cannot make a stipulation that contradicts the Torah.
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