Validity of contract

I was made to sign a blank contract (blank offer to purchase) by an agent named agent Leon Botha on the 29th September 2015 for a property I was initially interested in buying. This was despite informing the agent that I am a first time buyer and he assured me that he will lead me well in the process. He had subsequently assured me that he will get back to me within a day or 2 with a signed offer to purchase from the seller. When the time had lapsed I didn't hear from him and decided to pursue another property from a different agency. I then received the signed offer to purchase (dated 30 September) via email from Leon only a week later on the 6 October 2015. When I told him that I had already pursued a new property he told me that I am still legally bound to the contract because they have a first in line policy. Now Pam Golding is threatening me with lawyers saying that I am still legally bound to their contract and that I will be sued for specific performances and damages. They also want to hold me liable for attorney fees and client costs. I question the validity of the contract as I was not only made to sign a blank contract, I was also only informed a week later of e agreement