Respondents must pay to witnesses subpoenaed for testimony or depositions on their behalf the same fees for attendance and mileage as are paid in the United States district courts in proceedings in which the United States is a party, provided that, in the case of a deposition subpoena addressed to a party, no witness fees or mileage need be paid. Fees for witnesses must be tendered in advance by any respondent requesting the issuance of a subpoena, except that fees and mileage need not be tendered in advance where the Office of Enforcement is the party requesting the subpoena. The Bureau must pay to witnesses subpoenaed for testimony or depositions on behalf of the Office of Enforcement the same fees for attendance and mileage as are paid in the United States district courts in proceedings in which the United States is a party, but the Bureau need not tender such fees in advance.