Is expiation obligatory upon one who untruthfully says "yes" in response to a question intended to confirm information, such as "By God?", even though he did not explicitly swear by God?

If your colleague did not intend an oath by saying "Wallahi?" but rather sought confirmation, then no expiation is due from you, and you must repent from lying. However, if he did intend an oath, then your saying "Na'am" (yes) is considered an oath, and it is a yamin ghamus (a false oath sworn knowingly) and a major sin due to the presence of lying. There is no expiation for it according to the majority of scholars; rather, what is obligatory is repentance.