السلام عليكم ورحمة الله وبركاته احسن الله اليك "If something that is measured is sold or traded for something that is weighed, or vice-versa, then it is permissible even if possession takes place after parting" Manhaj as salikeen(kitab al buyu page 93)(kallamullah.com). Can you please explain?

Ustadh AbdulAziz al-Haqqan

If u mean assalam or assalaf, it is exactly what we do when we buy something online, you pay the money for that which u know exactly what it is and they deliver it to you later.