Daf Yomi - Yoma 46 - Camparing Milah and Avodah

In Shulchan Aruch it is paskened that when there is a ברית מילה on Shabbos, it is preferable that the same מוהל should perform the מילה and the פריעה. Rama disagrees and says that it can be split up and one can do the מילה and the other can perform the פריעה. His proof is from the עבודה in the בית המקדש on Shabbos, which can be done by many כהנים. The גרא says that there is a basic distinction between the two. Performing the עבודה on Shabbos is הותרה and that is why many כהנים can participate, however מילה is only דחויה and hence should be done by one. The Beis Halevi defends the Rama and explains that there is a difference between the עבודה of שחיטה which is הותרה and the עבודה of הקטרה which is only דחויה. The rationale behind this distinction is that שחיטה must be done on Shabbos for there is a mitzva to bring these קרבנות today, however regarding הקטרה, it actually can be done afterwards - after Shabbos, but since there is a concept of חביבין מצוה בשעתה, it is done on Shabbos. The Rama's proof can be from הקטרה which is דחויה and nevertheless performed by many כהנים. Look at שמועת חיים for further details.