The starting point is the Gemara's teaching that at the final judgment a person is asked first whether he conducted his business affairs faithfully, before being asked about Torah study, procreation, hoping for salvation, and wisdom — a striking sequence explored in Shabbat 31a:10 and read the same way in Beit Yaakov, Ki Tisa 31.
The Rambam grounds this in the concrete conduct of a scholar, describing one whose business dealings are in truth and faith, who says no when he means no and yes when he means yes, is exacting with himself in accounting yet forgoing towards others, so that he stands by his word, in Mishneh Torah, Human Dispositions 5:13.
The pesukim that anchor the theme in halachic honesty are Vayikra's demand for honest measures and balances tied explicitly to the Shemot in Vayikra 19:35-36, and Devarim's insistence on a complete and honest weight as a condition for enduring on the land, calling all dishonesty in this realm abhorrent to God in Devarim 25:13-16 — a theme elaborated at length by Sefer HaChinukh 258:1 regarding the mitzvah not to cheat in measures.
Kad HaKemach reads the verse as a warning to be pure in speech, never uttering anything but truth even in casual talk, and all the more so in one's business dealings, where one's yes and no must be reliably true, in Kad HaKemach, Faith 1:1.
Mesillat Yesharim develops the ethical-mussar dimension, teaching that God desires nothing but faithfulness, and stringing together verses that show God's guarding of the faithful and the demand that one's very hands be clean of anything that clings to them unseen, in Mesillat Yesharim 11:15.
Tomer Devorah broadens the frame from commercial honesty to the trait of truth itself, presenting Yaakov's descendants as those who deal only in true conduct even beyond the letter of the law, mirroring how God deals truthfully and mercifully with those who act with straightness, in Tomer Devorah 1:23.
Likutei Moharan connects business dealing to faith on a mystical plane, describing how binding one's inner thought to Torah during the very act of transacting business draws up sparks bound up with emunah, in Likutei Moharan 280:2.
Duties of the Heart situates faithful dealing within trust in God more broadly, teaching that even while relying on Divine decree a person is still obligated to pursue the ordinary means of his affairs rather than abandon effort entirely, in Duties of the Heart, Fourth Treatise on Trust 4:23.
Later mussar and chassidic figures extend the ethical treatment of faithfulness into prayer and inner conviction, as when Shiurei Daas ties acquiring true emunah to sustained reflection during the brachot in Shiurei Daas 3 p. 40, Rav Kook links bitachon in God's providence to the cultivation of moral character in Ein Aya, Berakhot 9:49, the Sefat Emet reads Yosef's reliance on God rather than his own stratagems in prison as a model of such trust in Sefat Emet, Bereishit, Miketz 25:3, the Tanya elevates charity as the quintessential active mitzvah expressing this same fidelity in Tanya, Part I; Likkutei Amarim 37:19, the Alshich reads Leah's giving over all the dudaim to Rachel as a lesson in relying wholly on God rather than natural means in Alshich, Rav Yosef Yaavetz distinguishes the faith of the disciple from the faith of the master in explaining the Israelites' belief at the sea in Rav Yosef Yaavetz, and Daas Torah identifies faithfulness as a quality of constancy and non-change, the same quality underlying trustworthy dealing, in Daas Torah (Rav Yerucham Levovitz).
On the halachic side, the principle that a bailee who transfers a deposit to another bailee remains liable, because the owner can say 'you are trusted to me under oath, but he is not,' shows how the demand for scrupulous faithfulness extends even into the technical laws of guardianship in Bava Metzia 36b:3.
