I do not understand, what your client did, but: If your client is right, then your invoice was wrong. In that case you'll have to cancel the original invoice and to reissue a new one with the proper tax codes, if you do not have configured the proper tax codes before, then you have to configure them first. If your client was wrong, you have properly declared the tax and you do not have the chance to get your full amount, then you owe the tax anyway in your tax return, you should not change the invoice or the tax codes and you have to post the difference as bad debt loss.