Since the amount has not been billed to the client, the following steps will remove the charge from the client-matter.

In AP Manager, locate the voucher that included the charge. Double click on it to open it.  Once it has displayed on your screen, click on Tools > Adjust Voucher. In the Distribution section, delete the distribution line that contains the client-matter charged. Add a distribution line for the amount (do not use a client-matter), and choose the appropriate GL account (usually an expense account code) for the distribution.