In the very beginning of the Quran God declares that the Quran guides. As in this verse, whenever performing sal t is mentioned inf q follows it. Hence believing and serving God are not completed without serving his creation. Inf q means giving out and it has different levels. Sufis do not restrict inf q to material charity only.
Material charity is the least part of generosity.
There are other dimensions to generosity. As Ismail Hakki states giving out differs in accordance with the spiritual attainment of the believer. The giving out for the rich person is to give from his wealth, for the Gnostic it means to give out from his gnosis and for the ascetic to give out from the pleasures of the nafs. This is important since giving out gnosis bestows on the spiritually needy people an everlasting life, which does not end with death. Islam is a comprehensive way of life.