"He said he would return the money tomorrow."
と is a postpositional particle that marks when someone says something. So, when you want to say that someone said something you would put (quotation)と言いました/言った。
彼は明日お金を返す(と言いました） - he will return the money tomorrow (that's what he said). The first part is just a normal sentence telling you that "he will return the money tomorrow", the と言いました part however lets you know that this was just a quotation of what he said. 返す is just a plain form of the verb you might know as 返します and means exactly the same thing - present simple/future tense of "to recieve" 返す isnt as polite as 返します tho.