Under the AICPA rule, a member could charge a contingent fee for an amended return if the member can demonstrate a reasonable expectation of “substantive consideration” by a tax authority. It is important to note here that not all fees that are not fixed in terms of amount are contingent fees. CPAs often quote the customer a rate within a range. As long as the nature of the fee is determined by the effort to be spent and not the result to be achieved, the agreement is not a contingent fee.
For example, quoting a client a charge in a range for performing an audit of a financial statement is not a contingent charge because, in this case, what the client pays depends on how the audit goes, how clean the client's records are, etc. Attorneys and tax accountants can work on a fee basis contingent related to IRS audits or challenges to original tax returns. That means your accountant can't prepare your return for a contingent charge, but you could manage an audit that way.