Baba Metzia 49a - Never Tell a Lie

There seem to be 3 different sources that the gemara uses for not lying:

1. The gemara in kesubos (chapter 2) uses the pasuk of מדבר שקר תרחק.

2. The gemara here in BM uses the drasha of שתהא הין שלך צדק ולאו שלך צדק.

3. Rashi on 48b in describing why one who retracts from a sale doesn't qualify as an עושה מעשה עמך and rashi on 49b in describing why a levi can assume that the ba'al habayis will follow through with his promise to give him teruma and ma'aser - uses the pasuk of שארית ישראל לא יעשו עולה ולא ידברו כזב.

1. It seems that each one of these sources applies to a different situation. The pasuk of מדבר שקר תרחק applies to a situation where at the time it is being spoken it is an outright lie - such as כלה נאה וחסודה when she is in fact not that at all (the machlokes there is only due to the special circumstance, but everyone agrees conceptually that in other situation where it isn't necessary to lie for shalom, it would be assur).

2. The drasha that the gemara uses here in BM of שתהא הין שלך צדק ולאו שלך צדק, which the gemara explains that one shouldn't be אחד בפה ואחד בלב applies as rashi says to a situation where at the time that one is making a commitment he is planning on breaking his commitment. This also includes as the Maharatz Chiyus points out from the Rambam (dei'os 2:5) refers to being disingenuous and acting tricky in their personal relationships. At the time one is saying something it is not an outright lie, but the impression implied from their speech doesn't match their intent.

3.Finally there is the more subtle category where one simply changes him mind and wishes to break his commitment. This is the case where r' yochanan and reish lakish argue about יש בדברים משום מחוסרי אמנה, and we pasken like r' yochanan that by a small gift where the other person expects you to follow through even without a kinyan, the breaking of a commitment would be a violation of שארית ישראל לא יעשו עולה ולא ידברו כזב. Rashi seems to understand that the entire machlokes is only in a case where the rates change and therefore one has a legitimate reason to retract, but in a case where the rates don't even change even reish lakish would agree that it is assur to retract. The Shulchan Aruch (C.M. 204:7) paskens like the rambam that for a small gift it is assur to retract whether or not the rates change (which is like rashi that this is the case of argument), but the sm"a quotes the rosh that if the rates change even r' yochanan holds that it is mutar for one to retract. It is interesting to note that the rambam doesn't cite the pasuk of שארית ישראל לא יעשו עולה ולא ידברו, and just says that אין רוח חכמים נוחה הימנו which implies that it is only a problem m'drabonon.