If your fiancé is a UK national, then he should write to his MP and have them approach the H/O. He should make it clear that the evidence sent was not considered. And send the documents they said were missing, that you provided, to show that he earns enough.
Also, write here and complain that the ECO who handled your application was negligent, explaining that you want to take this matter to court if there is no clarification on why you were turned down when you provided the information that they said was missing.
complaints@homeoffice.gov.uk
It will take time, but do not be put off. Your case is almost exactly the same as mine, in so far as someone did not look at all the information provided properly before making a decision, and in my case he also asked me to prove something will happen in the future, which is impossible to do.
Don't worry it will happen, it just takes time.