Your client cannot unilaterally change the terms of the contract.  Modification of the terms of the contract would require that both parties (you and the client) agree to the modification.You can sue the client for breach of contract; however, since this is an installment contract, your lawsuit would not be for breach of the entire contract, but only for breach of the installment in which the other party is not complying with the terms of the contract.  If the client pays the agreed upon amount of future installments, you would have to continue performing your duties under the contract because there would not be any breach of future installments. Installment contracts consider the performance by the parties of each installment.  If there is a breach of contract as to a particular installment, it does not breach the entire contract; only that particular installment.