Siman 156 Seif 1
1. Afterwards one should pursue his business since Torah that is not accompanied by work will eventually become nullified and cause one to sin since poverty can lead a person away from Hashem. However, one should not consider his business primary rather it must be secondary and one’s Torah study primary and then one will succeed at both. One must be honest in his business dealings. A person must be cautious not to mention Hashem’s name in vain since whenever Hashem’s Name is found, death is also found. One must be cautious not to take an oath even if it is true since 1000 cities belonging to Yannai the king were destroyed because they took oaths even though they fulfilled them. One should avoid establishing a partnership with a gentile out of concern that he will have to take an oath and the Jew will be in violation of לא ישמע על פיך . Some authorities are lenient with regards to establishing a partnership with gentiles nowadays since they do not take an oath to their idols. Although they mention the name of their god their intent is the creator of the heavens and earth and they just associate Hashem with other forces and this would not violate לפני עור since they are not warned against worshipping forces together with Hashem. It is permitted to have business dealings with them according to all opinions as long as it is not one of their holidays (see Yoreh Deah siman 147).