倍返し

ばいがえし【倍返し】

a return gift that is twice the value of the present originally received; a payment of twice the amount originally received.

～する give sb a return gift that is twice the value of the original present; repay 《a debt》 twofold.

●売買契約は成立していたが, 売り主は受け取っていた手付け金を倍返しすることで解約してもらった. The contract for the house had already been signed, but the owner was able to get it annulled by paying the buyer twice the amount of the down payment he had received.