Placement agency passing the buck

We used a recruitment agency to find sales staff for our business. After very detailed specifications were given to the agency, we started the process and choose a sales assistant from the candidates. At this point the agency ensured us that if there is any problems or if the sales assistant is to be replaced, they will sort our all admin and replace all staff, within a three month period. This was given in writing. We were very impressed with the efficiency of the agency. We paid the fee of R9 000. One month later we came to realise that the sales assistant is in actual fact not sales oriented or a strong sales person, but more of a PA person. We contacted the agency to request a new sales assistant, with strong selling s****s. The agency informed us that this is not their problem any longer and there is nothing they can do. Their solution was to contact our labour lawyer. They also refuse to refund us any monies as the sales assistant signed a commission structure outlay, which, in their opinion, is a binding Fixed Term Contract. Needless to say, we feel very let down and disappointed that the agency will take our money and then pass the buck.