All appeals are dismissed.

The costs of appeal between the Plaintiff and Defendant B are assessed against the Plaintiff.

The grounds of appeal are examined.

1. Plaintiff’s ground of appeal

A. Ground of appeal No. 1) Since advance payment received at the time of the contract for construction works has the nature of advance payment. Thus, if a contractor has to return advance payment due to reasons such as rescission or termination of the contract after advance payment, advance payment shall be naturally appropriated for the construction cost corresponding to the enhancement of the advance payment up to the time when the principal debtor has not expressed his/her intention of offset, barring any special circumstances: Provided, That in the case of a guarantee contract for return of advance payment, the guarantor guarantees the principal debtor's performance of his/her obligation to return advance payment, the occurrence and scope of reasons for the payment of such advance payment shall be determined based on the terms of the contract subject to the guarantee. Therefore, how to determine the contents of advance payment subject to advance payment should be governed by the agreement between the parties to the contract (see Supreme Court Decision 2002Da68362, Nov. 26, 2004; 200Da168362, Feb. 20, 200).