The Treasury Department letter is available here. It makes clear that the Treasury Department will allow an "eminent European" to oversee that EU-originating data is properly protected.
The working of the Terrorist Finance Tracking Program and the actual safeguards and controls of data are described in this document here.
The EU response is available here.
Interestingly, the French Government issued a statement on the matter authorizing the Council Presidency to sign the draft response above.
The Council issued a handy press release on the matter, available here.