pay someone/something back        
phrasal verb with pay verb uk/peɪ/ us/peɪ/ paid | paid
B1 偿还(钱);还(某人)钱
to pay someone the money that you owe them
  Can you lend me a fiver? I'll pay you/it back tomorrow. 能借给我5块钱吗?我明天还你。 
 - More examples
 - He still hasn't paid me back that money he owes me.
 - If I could borrow 20 pounds I'll pay you back next week.
 - She lent me some money last month and I forgot to pay her back.
 - They promised to pay me back and they never did.
 - I have borrowed money from people in the past but I've always paid them back.