1) One has a friend or brother who doesn’t pay khums but buys him clothes. Can he use those clothes in daily life / Salah (not sure with which money the clothes or stuff was bought halalharaam)

2) One assume he has cleared his Khums account but later realized it was wrong you corrected and paid off. In that duration heprayed with the clothes he bought thinking he has paid Khums. Does he have to repeat Salah?

1- Yes. You can even pray with them.
2- No.