I am a rental agent. I usually have clients sign an agreement stating they will pay me a one month fee if they rent the apartment I show them. Of course the one time I did not have a client sign there is a disagreement. I showed my client an apartment. He said he liked it but needed to discuss with his wife. He called me the next day and said that he had already signed the lease and said he will of course pay my fee of one month rent. After not taking my calls for several days, he called and said he will only pay a portion of my fee. He sent me a check for $200. The agreed upon fee was $1900. What can I do?