What should I do, as I swore to my friend that I would give him a thousand dinars if he was the speaker, and then it became clear to me that he was, and I am unable to fulfill this amount?

If a person swears an oath about something they predominantly believe to be true, and then it turns out to be otherwise, there is no expiation (kaffarah) upon them. This is considered part of "laghw al-yamin" (idle oath), which Allah has forgiven, and it is the view of the majority of Imams. Allah Almighty says: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what your hearts have earned." Scholars have mentioned that "laghw al-yamin" includes what is uttered unintentionally, or swearing an oath about something a person believes to be true, but which later proves to be incorrect.