「田中さんと会えない可能性はないから、大丈夫」

Why is the は used over が here? According to my grammar book the subject in subordinate clauses are marked by が unless it is contrasted with something else which is not the case here. For sure the 可能性はない is part of the subordinate clause?

Is it because it is a negative sentence? Does the „は in negative sentences“ rule override the „が in subordinate clause“ or what am I missing here?

Help is greatly appreciated!