Responsa for Nazir 5:11

Q. Before A and B entered, as equal partners, into a business transaction wherein B was to be the active partner, A said to B: "Give me your faithful word as a religious Jew that you will not deny me my share of the profits." B complied with A's request. When they came to divide the profits, A demanded that B take an oath to the effect that there were no other profits except those he had admitted. B claimed that he had already given his word to A, which is equivalent to an oath.
A. B must take the oath usually taken by all partners, which is administered by the hazzan holding the Scroll of the law. Although giving one's faithful word is also considered an oath, it is not as solemn as the oath administered while holding the scroll of the Law, and can not take its place.
SOURCES: Cr. 171; Pr. 606; L. 379; Mord. Shebu. 765; cf. Hag. Maim. Shebuoth 11, 3; Moses, Minz, Responsa 17.