There is a מח' ראשונים ואחרונים what is the reason for אין שליח לדבר עבירה. The תוס' הרא"ש here ד"ה הני as well as the סמ"ע in סי' קפ"ב ס"ק ב hold that the reason is a סברה because the משלח thinks that the שליח won't do the שליחות because דברי הרב דברי התלמיד דברי מי שומעים. However if he knows that the שליח will do the עבירה he is חייב because the סברא does not apply. The סמ"ע seems to hold that אין שליח לדבר עבירה is a problem in the appointment of the שליח. If I am not confident that he will do the שליחות then I am not really appointing him and he is not my שליח.
The ריטב"א in קידושין מ"ב and רעק"א here both say that it is a גזירת הכתוב and not a סברה. The ריטב"א says that the Gemara mentioned the סברה because it applies most of the time but even where the סברה does not apply, for example שוגג (see below), we say אין שליח לדבר עבירה in any case.
תוספות in a number of places argue with the ריטב"א and say that יש שליח לדבר עבירה where the שליח is a שוגג. This would seem to be like the סמ"ע, that since the שליח doesn't know it is אסור the משלח is confident that he will do the שליחות and therefore it is a good appointment. Similarly, the רמ"א writes (סי' שפ"ח סע' ט"ו) that if the שליח is הוחזק to do this עבירה you don't say אין שליח לדבר עבירה and the משלח is חייב. The reason would seem to be like the סמ"ע, that since he is הוחזק you believe he will do the עבירה and therefore you really are appointing him.
The gemara asks that an עבד ואשה since they don't have to pay we should say יש שליח לדבר עבירה and the Gemara answers that since if they get divorced or freed they have to pay that is called a בר חיובא. The Gemara's ה"א is very difficult. רעק"א and the רש"ש ask even if they don't have to pay they are certainly עובר the איסור of גזילה so how can the Gemara possibly think that they are called a לאו בר חיובא just because they won't have to pay back the money? Since they are violating the איסור we should still say אין שליח לדבר עבירה
The question would seem to be לשיטתו that אין שליח לדבר עבירה is a גזירת הכתוב and not a סברה however, according to the סמ"ע we can answer as follows. The ה"א was that since they won't have to pay, the משלח thinks that they will do the עבירה and therefore the מינוי is good. The Gemara answers that since they may eventually have to pay, the משלח is not confident that they will do it and therefore we would say אין שליח לדבר עבירה. The ה"א is very similar to where the שליח is a שוגג, since you think he will do the שליחות we say יש שליח לדבר עבירה