[souffle]

aww that's a shame. You can use a cotton ball dipped in cooled boiled water and gently wipe the area. Tissue dry then brush with a soft toothbrush.
Another thing to watch is the pouch on the side where the missing paw is. They do use the paw to push food forward and out the pouch sometimes so it can become impacted. Feel along there to make sure that it doesn't seem full all the time and that there is no drooling from the mouth on that side.
Some hamsters cannot use a wheel after an amputation as it breaks open with running so provide lots of alternative enrichment.