Is it permissible to reject a girl after performing Istikhara and feeling uneasy about her, despite her and her family's agreement, and is seeing her in the street considered a sharia-related transgression that requires expiation?

It is permissible for a suitor to look at his fiancée, as this fosters affection and ensures a lasting relationship. Neither her permission nor the permission of her guardians is required for this. He may send someone to look on his behalf, and it is permissible to repeat the looking until the purpose is achieved. There is no blame upon him if he does not marry her should he not feel comfortable with her. It is preferable for the looking to occur before the official engagement to avoid embarrassment.