What is the ruling on using sandals left at mosque entrances for ablution with the intention that their owner will gain reward?

The sandals placed at mosque entrances are endowments from their owners for anyone to use. This is an ongoing charity (Sadaqah Jariyah) for which the owner continues to receive reward even after death. As for sandals owned by some worshippers that are not endowments, it is not permissible to take them without the owner's permission.