We have a public adjusting firm and one customer entered into an agreement (written) with our firm to prepare, present & negotiate the settlement for a state farm claim for a fee of 25%. After receiving the 1st check for over $12k, they deposited the money, without our endorsement (we were listed as an additional payee), and does not want to pay us our fee.
If the sum/fee owed is for the balance of the contingent fee for the PA services, then you would have to file suit for breach of contract. The client may also have liability if they endorsed the name of your company on any draft without permission.