AUDITING-URGENT

based on the fact that this client is not paying the fee
there are a few factors that you must be aware

1. the client's firm might come across going concern problem

2. the client might be a risky client

3. the current year's audit engagement is at a high level of risk

what you can do is

1. mail the client with the accounts and check with the client/ client's staffs
and see if the client is aware of the unpaid account

2. since the unsettled fee has been accumulated for 3 audits so there is a very high chance of the client not paying for this coming audit too , the firm has all the right to refuse the engagement until the previous accounts has been settled.