Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Volume 13, Page 20, Fatwa No.15901
Question: Is it permissible for a Muslim to sell trousers and under garments for non-Muslim women?
Response: It is permissible to sell clothing to disbelievers, both men and women, if these clothes [completely] cover [the body from that which is Islaamically required] and do not include crosses (crucifixes), and specifically regarding men, that which does not contain silk. In principle, with respect to selling, all things are halaal until evidence suggests impressibility, even if it were for a Muslim or disbeliever.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.