Salam, After I urinate, I do Istibraa and try wait for the final pools of urine to come out, and then wash my male genital organ and put my pants back on.  Then later as I walk around or bend over, urine drops exit my male genital organ and obviously touch my underwear.  Given I have water on the male genital organ after Istibraa, can I deem the urine drops as Tahir? Or do I need to Taher and change underwear each and every time this happens (which is every single time I urinate)?