Is retrieving money from a friend—without his knowledge—after discovering his lack of urgent need for it and without the father's knowledge considered a sin or theft?

If you acted based on a reason that later proved to be non-existent, you are permitted to revoke that action, similar to a divorce conditioned on a non-existent stipulation. In this case, since he deceived you and lied to you, he must return the money to you, and you may reclaim it from him even without his knowledge, because he consumed your wealth unjustly.

However, if he did not deceive you and was genuinely in need, then the charity has been given, and you are not permitted to revoke it after he has taken possession of the money, even if he later becomes affluent, nor are you permitted to take the money without his knowledge.