I'm free at around 7PM.
Is it correct?
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There's nothing wrong with using the present indicative instead of future tense when nailing down an action to a time or speaking of things in the future as if they have already occurred. There is even a literary device known as prolepsis, which deals with exactly this.
How often have you heard someone in a movie or TV show say:
Reach for that gun and you're a dead man.
That is one example of prolepsis. Your example is another.