I seem to understand that the most stringent form of hiring a nochri on Shabbos would be through Kablanus since the nochri doesn't benefit more if he works harder whereas the Jew does. This seems incompatible with Rabeinu Tam's opinion that one should be allowed to hire a nochri for construction work through Kablanus.
shmuel greene, nj, usa
I am unclear regarding your statement "since the Nochri doesn't benefit more if he works harder, whereas the Jew does." Rabeinu Tam seems to explicitly contradict your statement in the beginning of Tosfos (DH "Arisa"), by explaining that if a Nochri is scheduled to finish a job at a certain time, i.e. in a year, the Jew could care less when he works on the property as long as he finishes the job. This is better than Arisa (which is also permitted according to Rabeinu Tam as he Paskens like Rebbi Shimon Ben Gamliel), and certainly better than a Schir Yom (which is forbidden) whose contract is essentially that he has to work on Shabbos.
All the best,