The gemara discusses the issue of paying 10 gold coins for robbing someone of their mitzvah. The gemara questions whether a mitzvah containing multiple brachos such as birchas hamazaon is 10 zehuvim per bracha, or 10 for the entire mitvah. Assuming it is per bracha, the gemara assumes the amount for birchas hamazon would be 40. Rashi explains that 10 per bracha, which means that the fourth Rabbinic bracha of ha'tov v'hamei'tiv would also have 10 zehuvim. Tosafos (d.h. v'chayvu) questions when one is called to the Torah and another grabs the aliya whether the latter owes 10 zehuvim. Tosafos offers two arguments why the person who "stole" the aliya wouldn't have to pay: 1. the rights to an aliya belongs to everyone (perhaps this is only when one was supposed to get the aliya, but wasn't yet called up?). 2. Even if a Yisroel stole the aliya of the kohein he wouldn't have to pay because וקדשתו - לכל דבר שבקדושה לפתוח ראשון ולברך ראשון, is only an אסמכת. Tosafos seems to hold that the requirement of וקדשתו is only d'rabonon, and since it's not from the Torah, there isn't any obligation to pay for stealing that right.Yet, in the very next Tosafos they seem to hold that one is required to pay 10 zehuvim for both ha'tov v'hameitiv and borei pri ha'agfen, both of which are only d'rabonon?
To me it seems that Tosafos isn't focusing on whether the mitzvah of וקדשתו is d'oraysa or d'rabonon. First of all, the magen avrohom (201:4) points out that the gemara in gittin 59b clearly holds that וקדשתו is d'oraysa. Secondly, even if it d'rabonon, there should be a requirement of 10 zehuvim. Tosafos fully agrees that the requirement to give kedusha to the kohein is d'oraysa. However, the Rabbonon came along and gave specific examples in which the kohein must be honored, one of the being the first aliya. But, both the Torah and the Rabbonon never gave this as a right and entitlement of the kohein, rather they demanded of the tzibbur to honor the kohein in this manner. The concept of paying 10 zehuvim is only when one steals a mitzvah that their friend is entitled to, it doesn't apply when one merely fails to fulfill their obligation to that person. Tosafos perhaps understands that had the Torah been referring specifically and directly to giving a kohein the first aliya, it would be understood not just as an obligation of the tzibbur but an inherent right of the kohein granted to him by the Torah, and if stolen from him he would be owed 10 zehuvim. But, since the specifics were only introduced by the Rabbonon, it is not an inherent right, rather they placed an obligation on the tzibbur. Therefore, even when the tzibbur fails to fulfill their obligation, no one would owe money to the kohein.