This question already has an answer here:
I'm aware of precisely one word that is spelled and pronounced the same, yet has a completely opposite meaning depending on its context: sanction.
On one hand, it is official permission for something.
On the other hand, it is a penalty (so, the opposite of permission).
Yet you could sanction (give permission for) a sanction (penalty to be applied). Hm.
Are there any other homonyms that share this trait of having directly opposing meanings?