27 C.F.R. § 70.144 – Special rules.

(a) Actual notice or knowledge. For purposes of 26 U.S.C. 6321 through 6327, an organization is deemed, in any transaction, to have actual notice or knowledge of any fact from the time the fact is brought to the attention of the individual conducting the transaction, and in any event from the time the fact would have been brought to the individual's attention if the organization had exercised due diligence. An organization exercises due diligence if it maintains reasonable routines for communicating significant information to the person conducting the transaction and there is reasonable compliance with the routines. Due diligence does not require an individual acting for the organization to communicate information unless such communication is part of the individual's regular duties or unless the individual has reason to know of the transaction and that the transaction would be materially affected by the information.
(b) Subrogation: Where, under local law, one person is subrogated to the rights of another with respect to a lien or interest, such person shall be subrogated to such rights for purposes of any lien imposed by 26 U.S.C. 6321 or 6324. Thus, if a tax lien imposed by 26 U.S.C. 6321 or 6324 is not valid with respect to a particular interest as against the holder of that interest, then the tax lien also is not valid with respect to that interest as against any person who, under local law, is a successor in interest to the holder of that interest.
(c) Disclosure of amount of outstanding lien. If a notice of lien has been filed (see § 70.148 of this part), the amount of the outstanding obligation secured by the lien is authorized to be disclosed as a matter of public record on TTB Form 5651.2 “Notice of Federal Tax Lien Under Internal Revenue Laws.” The amount of the outstanding obligation secured by the lien remaining unpaid at the time of an inquiry is authorized to be disclosed to any person who has a proper interest in determining this amount. Any person who has a right in the property or intends to obtain a right in the property by purchase or otherwise will, upon presentation of satisfactory evidence, be considered to have a proper interest. Any person desiring this information may make a request to the office of the Bureau named on the notice of lien with respect to which the request is made. The request should clearly describe the property subject to the lien, identify the applicable lien, and give the reasons for requesting the information.
(26 U.S.C. 6323)