The Rebbe's guidance on conducting business according to Torah:
Never compromise Shabbos for business. Shabbos observance is not to be compromised for convenience or financial gain, even where it seems beneficial (IK 5898). Buying one's own business and becoming independent is positive, provided Shabbos is kept and nothing of one's Yiddishkeit is sacrificed (IK 1436). The Rebbe also campaigned against communal Shabbos violations, such as ships departing on Shabbos (IK 4696).
Honesty brings blessing. Business conducted honestly, with the intention of using its fruits for Torah purposes, is itself elevated (IK 7380). If there is any possibility of having wronged someone, rectify it (IK 1758). Difficulties in business can sometimes be resolved through honest introspection, including about one's business dealings, together with checking tefillin and mezuzos (IK 1218, 3980). When Hashem grants success, expand to meet it, and make clear agreements with others (IK 3678).
Consult a rav on the halachic side. After acquiring a new business or property, have a rav address all the halachic matters involved (IK 6821). Interest arrangements need a heter iska; dealings with banks can be permitted when handled properly (IK 3129, 5045). Be careful not to encroach on another's livelihood (IK 5671). On loans versus sales and gifts, see IK 116.
Torah study for businesspeople. When a businessman sets aside time for Torah, especially when it takes effort, it is especially precious (IK 1006).
See also: פרנסה – חובות והלוואות; פרנסה – שותפות.