Is it permissible to donate trousers to non-Muslims, or must they be discarded, after I intended to replace them with skirts?

The preponderant view is that disbelievers are addressed by the ancillary rulings of Sharia. Therefore, it is not permissible to give away revealing clothes as charity to a disbelieving woman if it is known that she will use them to display herself in a forbidden manner, unless someone who can benefit from them in a permissible way is found. Otherwise, they should not be donated.