Answer
If a person doubts whether he has performed istibrāʾ or not and fluid comes out and he does not know whether it is pure or not, it is impure; and in the event that he performed wuḍūʾ, his wuḍūʾ becomes void (bāṭil).
If someone who has not performed Istibrāʾ becomes confident that no urine is left in the urethra due to the passing of time since he urinated, and if he then sees some fluid and doubts whether it is pure or not, that fluid is pure and it does not invalidate wuḍūʾ either.