For indeed we can redeem with it הקדש and מעשר, etc. - שכן פודין בו הקדשות ומעשר
A חומרא which is written in the תורה cannot be disregarded even if it is possible to say כ"ז אשים בק"ו. We cannot derive קידושי שטר from קידושי קרקע, because קרקע has the חומרא of שכן קונין מטלטלין אגבה.
A defender should become a prosecutor - סניגור יעשה קטיגור
The difficulty with כסף מכניס כסף מוציא is only if it is the same person (the קונה) who both makes and breaks the קנין by giving כסף; however if the קונה gives (to make the קנין) and receives (to break the קנין) that is understandable.
According to רש"י when a father sells his daughter for עבדות that is considered בע"כ; however when he marries her off that is not considered בע"כ (since [eventually] she would have agreed). According to תוספות the יעוד which takes place later is considered the בע"כ, but it is not considered אישות since the money was given for עבדות and not for אישות.