בבא בתרא מז:
"א"ר הונא תליוהו וזבין זביני זביניה זביני"
Rav Huna says if one “hangs up” and forces another to sell him his item, the sale is valid even though it was under duress. The מנחת חינוך [לח] brings the רמב"ם who paskens that although the purchase is valid, the purchaser is oiver לא תחמוד. The מגיד משנה asks, if so, why should the sale remain valid, we have a rule: "כל מילתא דאמר רחמנא לא תעביד אי עביד לא מהני"? ע"ש. The מפרשים discuss a few details regarding this rule of אי עביד לא מהני.
The [חוש"מ ס' ר"ח, סי' א'] רמ"א says that if one swears that he will not make a certain sale and he is עובר his שבועה and makes the sale anyway, the sale is still valid. The ט"ז wonders, since he is going against his שבועה and being עובר a לאו why don’t we apply the rule of אי עביד לא מהני and the sale should be בטול? The ט"ז explains that the only time we say this rule is if there is a איסור מצד עצמו. Here the איסור מכירה is because of a side issue that he made a שבועה, but the מכירה itself is not a מעשה איסור. This is also why the ברייתא that brings a case where a גנב/מוכר threw a בהמה into the חצר of the לוקח for him to be קונה on Shabbos the sale remains a valid sale. This is because the מכירה on Shabbos is only אסור because the יום is גורם, but the מעשה, in of itself is not אסור, therefore we do not apply the דין of אעל"מ. [שם בגליון ס' ר"ח] רע"א points out, that in the above case where he swore not to make the sale, although if he was עובר his שבועה and made the sale anyway it would be חל, if he made a שליח to do the sale, it would be בטול, because of "אין שליח לדבר עבירה".
Interestingly, [קובץ הערות ס' עד' אות ב'] ר' אלחנן explains that אעל"מ is only שייך when the איסור is on the חלות דין that is created from the action. For example, תמורה, the issur is the switching of Kedusha that is חל through the תמורה, so אעל"מ can say that this חלות דין does not take effect. However, when the issur is on the מעשה, for example "אותו ואת בנו לא תשחטו", where the issur is not the היתר that happens through the שחיטה, but rather the actual מעשה שחיטה is אסור, on this מעשה we don’t say אעל"מ, because it was done and how can we undo an action? This is also why a שחיטה done on Shabbos is כשר even though it’s a לאו, because the issur is not on the היתר that comes from doing the שחיטה but on the מעשה שחיטה and on that we don’t say אעל"מ.
The [שסא, אות ה'] מנחת חינוך says the only time רבא holds אעל"מ is when the לאו is on a specific מעשה. For example, לא יחליפנו ולא ימיר, the pasuk tells us that one does not have the "כח" to be מחליף. But when the לאו is said בדרך כלל like regarding שבת – "לא תעשה כל מלאכה" - we can’t learn the pasuk that one does not have the כח to do a מלאכה, because if he is oiver and builds a house or plants a vineyard, the house is standing and the vineyard is planted! Rather the תורה is saying לכתחילה do not do this מעשה. On this type of לאו we do not say אעל"מ