Migu with Ulterior Motive for not Lodging the Alternate Claim
One of the premises of a successful migu is the symmetry between claims. Since the litigant could just as easily have offered a better claim, we trust the validity of the ACTUAL claim. If he were a liar he would probably have offered a legally "superior" lie. What would happen to the migu is there were legitimate reason for the non-proposal of the alternate claim?
The most common instance concerns a situation of migu de-ha'aza - where the alternate claim is more audacious than the original claim. Since the alternate claim would demand greater courage, we assume it was not tendered because the litigant was afraid and NOT because he is claiming truthfully. Many Rishonim reject this form of migu (see Nimukei Yosef in Bava Metzia (2a) and the Rabbeinu Yona in Bava Batra (30b) They base themselves on Gemara in Bava Metzia (3a): Rava questions the obligation to swear in instances of modeh be-miktzat. They interpret Rava's question as a secondary one not a primary one: Rava was not denying the fundamental logic of a shevua for someone who partially confesses to a claim; independently, the shavua is logical. However, since the litigant had the option to flatly deny the claim he should be exempted from the shavua because of a migu: "believe me that I only owe fifty percent, for if I wanted I could have denied the entire claim and been excused from a shavua." Rava's answer may be denying migu in this instance since flatly denying a debt requires greater courage than merely confessing to partial debt. This reading of the Gemara would disable the potential migu in the question of the Gemara because of the "asymmetry" between the claims noted in the conclusion of the Gemara. Since the alternate claim demands greater boldness it is considered a migu de-ha'aza which is an inoperative migu.
By contrast, many Rishonim deduce from a Gemara in Bava Batra (36a) that even a migu de-ha'aza is acceptable. In this Gemara a person, whose food was constantly being eaten by animals, seized those animals and demanded payment for his eaten food. He was allowed to claim a debt up to, and including, the worth of the seized sheep since he could have hypothetically claimed that he had purchased the sheep; this migu empowers his statement as to the amount of debt owed him. Presumably, this is a classic migu de-ha'aza since it takes greater courage to claim purchase of the sheep than it does to claim a debt of damage. Despite this discrepancy, the Gemara validates this migu, and we may infer the validation of similar asymmetrical migus.
In all likelihood this machloket about whether to endorse a migu de-ha'aza surrounds Rav Elchanan's original inquiry regarding migu. If migu is a psychoanalytical tool to determine honesty it would utterly fail in this instance; we can supply ample explanation for the avoidance of the 'superior' claim. However if migu is a legal force and awards victory to the litigant who could have legitimately victored through alternate means, we may be more inclined to consider migu as long as the alternate option was indeed legally available, even if it was unlikely that the litigant would have felt comfortable choosing that option. Migu is not based on the personal psychology of the litigant but rather on the legal reality that alternate victorious options exist.
An interesting logic may emerge from the comments of the Ran to Bava Metzia (3a). Explaining Rava's question and answer about modeh be-miktzat slightly differently, the Ran claims that migu de-ha'aza is acceptable (based in part on the aforementioned Gemara in Bava Batra). Though a migu de-ha'aza is acceptable in general, it cannot supplant an obligation to swear. Why should a migu de-ha'aza be effective in general but inadequate to replace a shevua. Said otherwise, how can we reconcile a system in which a GENERAL migu replaces a shevua and migu de-ha'aza is accepted (according to these Rishonim as a legitimate migu), yet a migu de-ha'aza fails to replace a shevua!!
Perhaps this position is based upon the following perspective: typically migu contains BOTH elements which operate in a complementary fashion. Where one element of migu fails (under certain circumstances) the other element is still available. Migu is BOTH a barometer of honesty as well as a legal force awarding the money to the most leveraged person. Certain areas of halakha may mandate a PARTICULAR aspect of the migu; for example migu's ability to replace a shevua may be feasible only if a migu operates as a barometer of truth. This beirur can then exempt a shevua which itself is a truthtest. Migu de-ha'aza even if operative can only function as a "legal force." We cannot determine his honesty based on his non-proposition of the better claim since he was presumably too frightened to pose such an audacious claim. However we still embrace migu de-ha'aza since it operates as a legal force (ko'ach) since had he offered the alternate claim he would have triumphed. However, a migu de-ha'aza which can only be considered a ko'ach cannot supplant a shevua which demands beirur or at least a fully operative migu which can provide a form of beirur; migu de-ha'aza cannot function in this capacity and hence, though acceptable (as a ko'ach in general situation) it cannot exempt a shevua.
Perhaps the inverse to a migu de-ha'aza can be located in an interesting Mishna in Ketuvot (12b). The Mishna describes a dispute between a newly married couple regarding her status at the time of the wedding. The husband claims that she entered as a non-betula and the ketuva contract should be voided (or adjusted). She claims that she was raped after the engagement which would allow her to maintain her full Ketuva. Rabbi Eliezer accepts the woman's claim whereas Rabbi Yehoshua endorses the husband's. The Gemara presents a migu as the basis of Rabbi Eliezer's endorsement of the woman's claim. Since she could have claimed that she lost her virginity in an unrelated accident and she chose to confess to a rape (which occurred after engagement) she is believed. This is a peculiar migu since the migu claim is not LEGALLY more powerful that the original claim. Even is she claims this accident we may not believe her against the husband's claim of pre-engagement loss of virginity. It would appear that for this very reason Rabbi Yehoshua rejects this migu, argues with Rabbi Eliezer, and supports the husband's claim. Evidently, Rabbi Eliezer allows a migu as long as the alternate claim was more ATTRACTIVE, even though it is not LEGALLLY SUPERIOR. It would have been "easier" for her to describe a personal accident rather than admitting to rape. The fact that she did not choose the "easier" approach indicates her honesty. Presumably, if migu functions as a barometer of honesty it would function in this circumstance. However, if migu serves as a legal force awarding victory based on potential legal alternatives, it would have no applicability to a migu type which would not enable greater legal force than the original claim.
In essence, this migu known as migu she'eino gamur (a very poor and abstract phrase for this migu) is the logical inverse of a migu de-ha'aza. In this instance migu may function as a honesty barometer, but it certainly cannot function as a legal force since the migu claim is not superior. A migu de-ha'aza, by contrast, would probably function as a legal force since, had he lodged the alternate claim he would have succeeded. However, it cannot function as a barometer or honesty since there is logical explanation for his not lodging the alternate claim.