What is the ruling on investing money in building fences for Jewish cemeteries, and what should a Muslim do with the capital and profit if it becomes clear that this is unlawful?

The fundamental principle in transactions is validity and permissibility. There is no legal (Sharia) evidence that prohibits building fences around graves, as long as they do not include symbols of disbelief (kufr). The Hanbalis have explicitly stated the permissibility of a Muslim digging a grave for a Dhimmi (a non-Muslim living under Islamic rule) for a wage. Ibn Taymiyyah reported from Imam Ahmad the permissibility of this, while disliking the building of a "nawus" (a specific type of tomb) belonging to the Zoroastrians, because a grave is not specific to their false religion, unlike the nawus.