Thanks and it seems I have to wait for about 10 hours before being able to send again to same user and that’s great but it seems I can’t send the karma on the specific post but rather in the users profile right ?
You are sending karma for an exact post even if it can be checked by admin only. If any significant karma misusing, he can check and revoke karma (both positive and negative) which was sent abusively.
So if you don't say it in words, no one except admin will know for which post did you give karma, but you still do it for some exact post.