To Refund, To Reimburse
Transitive verb and Godan verb
はらいもどす
恐れ入りますが、お客様は鍵を失くされたので、保証金の払い戻しは出来かねます。
I’m afraid that we cannot refund your deposit since you lost the key.
払い is a "payment" and 戻す is "to return." If you return a payment, you are going to repay or to refund someone for their payment.
The readings come from 払い and 戻す.