Abandoning the Better (Tark al-Awla)
Abandoning the better or tark al-awlā (Arabic: تَرْک الأوْلی) is to do something good and abandon what is better to do. Shiite scholars do not take "abandoning the better" as a sin, and thus, they hold that prophets and Imams (a) might sometimes abandon the better. They maintain that in cases in which prophets and Imams (a) practiced istighfar (asking God for forgiveness), they did so because they had abandoned the better. In some verses of the Qur'an, some mistakes are attributed to some prophets. Exegetes of the Qur'an hold that since prophets are infallible, such mistakes are not sins from the standpoint of jurisprudence, rather they are "abandoning the better" (tark al-awla), that is, they just abandoned what is the better action to do.
"Abandoning the better" (tark al-awla) takes place when one has two options; one is good and the other is better, but they do what is good and abandon what is better. The term was not used in the Qur'an and hadiths; rather it was used by Muslim scholars. 
Difference with Sins
According to Tafsir-i nimuna, a sin is an action that counts as wrong for everyone, but "abandoning the better" is not wrong and is even favorable, but it is not good for some people to do because of their positions. For example, if a worker donates one day of his or her salary for the construction of a hospital, he has done a good thing, but if a wealthy person donates the same amount of money for the construction of a hospital, he will be berated, because he is expected to pay more, even though he has done something good in itself. Such an action is called "abandoning the better".