In Mitzvos #30 and #227, we spoke about the prohibitions against swearing unnecessarily and falsely, respectively. Here, however, we have an obligation to swear! The Talmud in Shevuos (35b) tells us that just as God has told us not to swear in certain circumstances, He has obligated us to do so in others, for example when required in a court case. Jews are not to swear by any earthly object (such as, "On my mother's grave!"), but only by God Himself.
It might seem strange that we should swear by God in something as mundane as financial matters, especially seeing how averse we are to swearing at all under normal circumstances. If anything, that's kind of the point! The very fact that we are swearing at all shows how honest and serious the matter is. By invoking God, we are actually bringing His blessing into the matter.
The Ramban (Nachmanides) is of the opinion that swearing is always optional and one is never obligated to do so.
This mitzvah applies to men and women in all times and places. It is discussed in the Talmud in the tractates of Nedarim (8a), Sanhedrin (63b) and, of course, Shevuos (35a-b). It is codified in the Shulchan Aruch in Yoreh Deah 237. This mitzvah is #7 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.