Try phoning and asking for your money back and find out on what basis they will not return it to you. If they simply tell you their legal rights, that you voluntarily gave them this money and that they have no requirement to pay you back then discuss this further with them.
Explain to them that this is not a legal issue but rather a spiritual and moral issue and on what basis biblically do they feel they have no obligation to return the money to you even though you are asking that they do so. If you feel that they took your money through deceptive means and promises then tell them so.
I believe that the true christian would return money given freely to them if the giver had a change of heart and felt he was truelly being wronged by. The true christian would not want to be the cause of stumblying this man or be the cause of ill feelings. The true christian would also know that they were not dependant on this man's money but would rather put faith in the fact that God will provide.
Let's say there was a man who donated half his land and built a Kingdom Hall on it then titled to deed over to the local congregation. One day in the future he is disfellowhipped because he is not considered repentent enough. The man is shunned by all the JWs and his parents move out of his house. They continue to use the hall which is right next to the house. This man feels dishonored and used and asks for the congregation to return ownership of the land and hall back to him. Although they have no legal obligation to do so and do believe that the right thing to do would be to return it to him or to appropiately compensate him.
Unfortunately, religious organizations aren't generally interested in doing the Godly thing when money becomes involved.
If you do call, it would be great if you could record it.