Is it permissible for a Muslim to keep 75% of bank interest and donate the rest, taking into account that banks deal with both the lawful and the unlawful?

The questioner's statement that banks generally deal with both lawful and unlawful matters is incorrect because bank interest is derived from usurious contracts, making it entirely unlawful. Even if a bank invests funds according to Mudarabah (definition) (profit-sharing) and its investments are predominantly lawful, it is still not permissible to deal with it, as this involves aiding in sin and consuming unlawful wealth. Donating a portion of the interest does not benefit.