In this week’s parshah the Torah details all the begadim that the kohanim were to wear. Several garments were comprised of wool and linen materials. Although generally such a garment would be prohibited to wear, it was permitted for the kohanim to don these garments.
There is a machokes Rishonim whether a kohen was permitted to wear his garments containing shatnez even after he was finished performing the avoda, or only while he was performing the avoda. The Rambam (Hilchos Kilayim 10:2) states that a kohen was only permitted to don his shatnez garments while he was performing the avoda. If he would wear them at any other time he would have transgressed the prohibition of wearing shatnez.
The Raavad there disagrees and posits that a kohen may wear his garments at any time provided that he remains in the Mikdash.
The Chasam Sofer (Teshuvos Yorah Deah 69) asks the following question on the Rambam’s opinion: The Gemara in Gittin 54b says that if a kohen had the wrong intentions while sacrificing a korban on behalf of someone else, rendering the korban invalid, the kohen must reimburse the owner for the value of the animal. If a kohen had the wrong thoughts while sacrificing an animal, he is not considered to have been performing the avoda; rather his actions were mundane. According to the Rambam when a kohen wears his garments while he is not performing the avoda he is transgressing the avera of shatnez, which is punishable by lashes. Why then would a kohen who has the wrong intentions while sacrificing a korban be liable to reimburse the owner for the value of the animal? The halacha is that if one does an action which obligates him to pay money and at the same time obligates him in the punishment of lashes, he will only receive the lashes and not have to pay. So, why would a kohen have to pay for the animal if he will be deserving of lashes.
The Chasam Sofer answers that one of only prohibited from wearing a garment that contains shatnez if he does so while wearing the garment for pleasure purposes. For example one may try on a garment for size, even if it contains shatnez. A kohen wears his garments as his uniform that he must wear when serving in the Beis Hamikdash. He is not wearing them for any benefit. Therefore when the Rambam said that a kohen who wears his garments while he is not performing the avoda he will receive lashes was referring to wearing them when he is not acting in any way for any service of the Beis Hamikdash. However, if a kohen is sacrificing a korban, even if the korban is rendered invalid, he is not wearing his garments for pleasure purposes. Therefore he is not transgressing the prohibition of shatnez even when he is sacrificing an invalid korban. Thus, he will be liable for reimbursing the owner for the value of the animal.
Based on this explanation the Dovev Meisharim (3:15) answers another question (asked by the Minchas Chinuch mitzvah 99:4): Why was the kohen not obligated to put tzitzis on the me’il (robe), which had four corners? A garment is only obligated in tzitzis if it is worn for pleasure. A uniform that one must wear for service is not obligated in tzitzis. Based on the Chasam Sofer’s explanation we can explain that since the kohanim did not wear their garments for pleasure, rather for service, they were not obligated in tzitzis.
However, the Rambam’s wording implies a different understanding than the explanation of the Chasam Sofer. The Rambam writes in Hilchos Klayim (10:2) that the reason why a kohen may wear his shatnez containing begadim is because wearing them is a mitzvah. The implication of this Rambam is that wearing the garments is a mitzvah and therefore we can apply the concept of assei doche lo sassei. Therefore a kohen may wear his garments only during the avoda in the Rambam’s view because otherwise he is not fulfilling the mitzvah of wearing the garments.
Based on this we can suggest that when a kohen has the wrong intentions while sacrificing a korban, even thought it is not considered an avoda, he is nevertheless fulfilling the commandment of wearing the garments. Therefore he will not have transgressed the prohibition of shatnez since we can still apply the halacha of assei doche lo sassei.