This can come under the purview of professional ethics as undercutting of fees.
However, if the engagement shows that the reduction in fees is due to reduction in quantum of work, then it cannot be said that the firm is guilty. The firm has to show that the fees was quoted low due to reduction in quantum of work (eg : there were less number of transactions during the year and such reduction in quantum of transaction is permanent in nature, etc). Some relationship has to be shown for low fees so quoted.