Bekhorot 34B

ונקצצה ⁠— טהור, קצצה מתכוין ⁠— רבי אליעזר אומר: לכשיולד לו נגע אחר יטהר הימנו.

and it inadvertently got excised, that individual is ritually pure. If he excised it intentionally, in an attempt to render himself ritually pure, the rabbis impose a punitive impurity upon him. The tanna’im disagree with regard to the extent of the penalty: R' Eliezer says: When he will develop another leprous mark, his status of ritual purity is assessed based on this new mark. When that mark is ruled ritually pure, he will become ritually pure from the ritual impurity generated by the mark he excised.

וחכמים אומרים: עד שתפרח בכולו, או עד שתמעט בהרתו מכגריס!

And the Rabbis say: The development of a new leprous mark is insufficient. Rather, he must wait either until the tzara'at of the new mark spreads over his entire body, a sign of ritual purity (see Leviticus 13:12), or unless his old baheret had shrunk to a size smaller than that of a split beanbefore he excised it. Otherwise, he remains ritually impure. In any event, it is clear from this Mishnah that R' Eliezer does not impose an indefinite penalty on one who transgresses. If so, why does he impose an indefinite penalty in the Mishnah discussing one who causes a blemish on the ear of a firstborn offering?

רבה ורב יוסף דאמרי תרווייהו: כי קניס רבי אליעזר בממונו, בגופו לא קניס.

Rabba and Rav Yosef both say, in answer to this question, that when R' Eliezer penalizes one who transgresses, that is only in a case which involves his property, e.g., a firstborn offering. But in a case which affects his body, e.g., tzara'at, he does not penalize him to such an extent.

ממונו ⁠— איכא למימר דאתי למיעבד, גופו ⁠— מי איכא למימר דאתי למיעבד?

The Talmud elaborates: With regard to his property, i.e., the firstborn offering, if he would not be penalized indefinitely, there is reason to say that he might come to perform an action which would cause a blemish. Since the firstborn offering is anyway prohibited for personal use until it is blemished, he suffers no risk in blemishing it. A greater deterrent than usual is therefore needed to prevent such action. By contrast, in the case of tzara'at on his body, is there a basis to say that he will come to perform an action and remove the white spot? In this case, the limited penalty of having to wait for purification from another leprous mark serves as a sufficient deterrent, as there is no guarantee that another mark will ever appear.