Owner driver contract cancelled unrelated event

We signed a service delivery contract as a owner driver with Cashbuild. When we signed the we werealready doing construction jobs at the time and made regular use of the store in Benoni to buy the materials and was approached to do there deliveries as well as we had a truck and they needed vehicles. They are now claiming that work is not done as a client on the construction end went to Fanie to complain as they are friends. Since the contract started in February the branch as over exceeded the target because of the extra work that we do for them but was told this morning that we will be working notice. No prior warning and the fact that the client complaining was at the store to complaining about repairs to his house and not a delivery from Cashbuild. They have no bases to then cancel the contract as the matter does not involve them. The contract with Cashbuild is a legal binding contract and delivery was met without fail.