Sinning for one's own eventual merit: Pesachim 15a, 20b-21a; Menachot 48a-b
Sinning for the merit of another, where both sin and merit come through the same item: Menachot 48a
Sinning for the merit of another, where the merit will be completely separate from the item on which the sin has been committed: Menachot 48a-b
Sinning on Shabbat, so that another might gain merit after Shabbat: Menachot 48a
Would a righteous person prefer to violate a light prohibition in order to prevent an unlearned layperson from violating a more serious prohibition, or not: Eruvin 32b