The Gemara says that we make the Kohen burning the Para Aduma (the red heifer) impure and then dip him in the Mikva.
The reason given is that there is an argument between the Rabbis and the Sadducees whether one may burn the Para Aduma in a state of Tvul Yom (after going to the Mikva but before nightfall). The Rabbis said that one may, while the Sadducees said one may not. Therefore, to show that we don't follow their opinion, the Rabbis forced the Kohen to be a Tvul Yom.
However, Tosfos says there that they didn't make him a biblical Tvul Yom, just a Rabbinic.
My question is how this helps. The Sadducees didn't believe in Rabbinic enactments. So this Para Aduma would be Kosher even according to their opinions (he wasn't a Tvul Yom as he was never impure). What's the point of making him a Tvul Yom?