Gittin 39A

הא מני ⁠– רבי מאיר היא, דאמר: אין אדם מוציא דבריו לבטלה.

The Talmud answers: In accordance with whose opinion is this baraita? It is in accordance with the opinion of R' Meir, who says: A person does not make a statement of consecration for naught. If one declared an item consecrated, even if he did not use the correct formulation, his statement is interpreted in a manner to render it meaningful. Therefore, although the master did not say that the slave is consecrated with regard to his monetary value, his statement is interpreted in this way. However, Rav holds in accordance with the dissenting opinion of the Rabbis. In their opinion the slave is not consecrated.

הכי נמי מסתברא, דקתני סיפא: וכן הוא, שהקדיש עצמו ⁠– עושה ואוכל, שלא הקדיש אלא דמיו. אי אמרת בשלמא רבי מאיר היא ⁠– שפיר;

The Talmud comments: This too stands to reason, as it teaches in the latter clause of the baraita: And so too, a freeman who consecrated himself works and is sustained from his labor, as he consecrated only his monetary value and he did not consecrate his body. Granted, if you say that this baraita is in accordance with the opinion of R' Meir, then the entire baraita works out well. According to the opinion of R' Meir, even when he consecrated himself, his statement is interpreted so that it is referring to a type of consecration that is meaningful.

אלא אי אמרת רבנן היא, בשלמא עבדו ⁠– לדמי קאי, אלא איהו ⁠– לדמי קאי?!

Rather, if you say that it is the opinion of the Rabbis, granted, the first clause of the baraita that deals with one who consecrates his slave is understood, as his slave exists primarily for monetary value. It is logical that when the owner consecrates him he intends to consecrate his monetary value. However, in the case of the latter clause of the baraita, which is referring to one who consecrates himself, does he exist for monetary value? Therefore, the baraita must be in accordance with the opinion of R' Meir.