In this case, the seventy dollars is a single block of money, so we generally refer to it in the singular. "The seventy dollars you owe me is ..."
Arguably this violates normal rules of grammar. Obviously "dollars" is plural and "seventy" makes it clear that it is more than one. In most cases we use a plural even if we are referring to the "things" as a group. "The three dogs who attacked me are .." not "The three dogs who attacked me is ..." "The three packages in the shipment are ..." not "is", event though it's one shipment. Etc.
Note this is an issue of grammar and not of fact. Like if I reworded the sentence to say, "The pack of three dogs that attacked me is ...", "is" would be appropriate, because now the subject is the singular word "pack", even though the pack has three members. Similarly "The shipment of three boxes is ..."
Off the top of my head I can't think of another example where a plural word is treated as a singular other than when talking about an amount of money. But I wouldn't say there are no other cases, maybe I'm just not thinking of them.