29 USC 1106 - Prohibited transactions

(a) Transactions between plan and party in interest
Except as provided in section 1108 of this title:

(1) A fiduciary with respect to a plan shall not cause the plan to engage in a transaction, if he knows or should know that such transaction constitutes a direct or indirect

(A) sale or exchange, or leasing, of any property between the plan and a party in interest;

(B) lending of money or other extension of credit between the plan and a party in interest;

(D) transfer to, or use by or for the benefit of a party in interest, of any assets of the plan; or

(E) acquisition, on behalf of the plan, of any employer security or employer real property in violation of section 1107 (a) of this title.

(2) No fiduciary who has authority or discretion to control or manage the assets of a plan shall permit the plan to hold any employer security or employer real property if he knows or should know that holding such security or real property violates section 1107 (a) of this title.

(b) Transactions between plan and fiduciary
A fiduciary with respect to a plan shall not

(1) deal with the assets of the plan in his own interest or for his own account,

(2) in his individual or in any other capacity act in any transaction involving the plan on behalf of a party (or represent a party) whose interests are adverse to the interests of the plan or the interests of its participants or beneficiaries, or

(3) receive any consideration for his own personal account from any party dealing with such plan in connection with a transaction involving the assets of the plan.

(c) Transfer of real or personal property to plan by party in interest
A transfer of real or personal property by a party in interest to a plan shall be treated as a sale or exchange if the property is subject to a mortgage or similar lien which the plan assumes or if it is subject to a mortgage or similar lien which a party-in-interest placed on the property within the 10-year period ending on the date of the transfer.