As mike_no wisely told you, this isn't a risk. Air inactivates the virus. So, exposing your finger is not an HIV transmission risk.
Below are the only risk for HIV transmission in adults:
1. Unprotected vaginal or anal intercourse
2. Sharing needles intravenously to inject drugs with other people.
Since your risk is not mentioned in the above two situation, you had no risk.
To deal with your 'what-if' see a counselor, they can help you with your anxiety.
Do you have a question?
It's not clear from your post.