pay back with interest：加倍回敬，连本带利。interest：n.兴趣；嗜好；利息；股份。

【例句】
An industrial firm issues bond to collect money and will pay back with interest.
工厂发行债券集资并偿还利息。

When borrowing money and the agreement includes paying interest then always pay the money back on time with interest.
借了钱如果原定合约包括了付利息时就按照条件本利一起依时清还。