I have heard that Rashi said that there are three requirements for conversion: brit milah; mikveh; korban in Beit Hamikdash. If this is true, why is it that this is not rendering modern conversions invalid until the Beit Hamikdash is rebuilt?
Bamidbar 15:14 says that a convert must bring an offering "throughout your generations". Sifre Zuta says that the inclusion of this phrase teaches that converts are accepted at any time. Haamek Davar states that the convert or their descendant must bring an offering when the Temple is rebuilt.
This question is related to another and I believe the answers are the same. The sugya (discussion) is found in Talmud Bavli, Krisus 8b-9a. The resolution there is essentially that the korban (offering) is not m'akev (does not invalidate the conversion), which is learned from a gzeiras hakasuv (exegetical interpretation of a verse).