What is the ruling on selling musical instruments for someone who has decided to abandon the profession of music after knowing it is unlawful, in order to use the proceeds for another project?

What you have decided regarding abandoning the music profession is correct, and you must destroy musical instruments. It is not permissible to sell or donate them, as they are intended for unlawful amusement. That which is forbidden to use is forbidden to sell, and this is the view of the majority of scholars. We advise you to fear Allah and seek lawful sustenance, for "And whoever fears Allah - He will make for him a way out and will provide for him from where he does not expect."