According to the Mishna (47a), the remnants of the blood of a korban ḥatat – a sin-offering – that was brought in the inner sanctum of the Temple, was poured out on the foundation of the altar that stood outside of the Temple, in the Temple courtyard.
Of the various sin-offerings, those that were brought by the High Priest (see 4:3-12) and those that were brought by the Sanhedrin on behalf of the entire community (see 4:13-21), as well as those brought on Yom Kippur (see 16:3, 15) were brought on the inner altar; the others were brought on the altar that was outside in the Temple courtyard (see, for example, the sin-offering of the king, Vayikra 4:22-26 or an individual sinner, Vayikra 4:27-35). In the case of the korbanot that were brought on the inner altar, sprinkling the blood – which, as we have learned, was an essential part of the sacrificial service – took place within the confines of the Temple. Nevertheless, what was left over after the blood was sprinkled was poured out on the altar that stood outside of the Temple, in the courtyard.
In searching for a source for this law, the Gemara on today’s daf quotes a passage from Sefer Vayikra that appears regarding both inner sin-offerings and outer sin-offerings (see 4:7, 18 and 25), which teaches that this is where the blood should be poured as it is the first place that the kohen reaches when he leaves the inner sanctum.
Some manuscripts of the Gemara do not include this last explanation, that the outer altar is the first place that the kohen reaches when he exits the inner sanctum, and it appears that Rashi did not have these words in his Gemara. Tosafot explain that Rashi understood that the source for this law is the language of the biblical passages themselves, and that it is not because ein ma’avirin al ha-mitzvot – that a person should not miss an opportunity to perform a mitzva. Tosafot argue that the idea of ein ma’avirin al ha-mitzvot only applies in situations where two mitzvot need to be fulfilled and we need to determine which should be done first. In cases where there is a single mitzva to do, we need separate instructions about what should be done.