Deputy Ferbrache is a consultant with the firm, according to his declaration of interests.
On Tuesday, the four other members of the committee and its chief secretary briefed Policy and Resources on a case taken on by the Public Trustee, a body Economic Development is politically responsible for.
'As part of this case, Ferbrache & Farrell, a local law firm, has to date been paid some £300,000 from the public purse as a result of work carried out for the Office of the Public Trustee,' P&R said in a statement.
'While the Public Trustee alone makes decisions about whether to take on a case, they are required to seek approval from the Committee for Economic Development if any case is likely to accrue costs in excess of £20,000.'
There is still a possibility the spending could be recovered from the trust's assets.
The Public Trustee has resigned.
'The unanimous view of the Policy & Resources Committee was that the circumstances could give rise to the perception of a potential conflict of interest for Deputy Ferbrache'.
It advised Economic Development to act swiftly.
'We understand the vice-president of the committee and the chief secretary met with Deputy Ferbrache on the evening of 21 November, following which conversation Deputy Ferbrache resigned his position.'
Deputy Ferbrache has yet to comment officially to the Guernsey Press, which approached him on the subject yesterday afternoon and again this morning.
He has, though, spoken to other media to strongly refute any conflict of interest.