Is it permissible for a fiancé, whose marriage contract has been solemnized, to bring forward his wedding date due to prostatitis, knowing that his financial circumstances do not allow for it?

If you are unable to expedite marriage, then you must be patient and practice abstinence. Among the things that help with this are: fasting, staying away from everything that incites desire, strengthening your connection with Allah and relying on Him, and occupying your time with beneficial deeds.