Answered by Sidi Mostafa Azzam
At what point does water become used if purifying clothes of intangible filth? When it comes off the clothes, or as soon as it touches the spot?
The water becomes used upon separating from the washed spot, just as in wudu’. (This is, of course, provided that the water purifies the spotwhich is the case when washing intangible filth–since if it does not purify the spot, the water becomes filthified, not used).