I can't understand the reference here. Is there such a thing as 'waste money， 废钱' in China? Or is it 'abandoned， discarded money'? Who threw the money there '我们‘ or 其他人？？ Is he alluding to some story or 成语？
那还能不同意？ 这就好比我们家后院扔着一堆废钱，你要拿它给我换一辆汽车。 我不光同意，甚至还有点儿不敢相信呢！
I translate: That's like someone threw a pile of money in our backyard and then you come along and offer to give me a car in exchange for it.
(Seems a weird idea, because first you need to know how much money it is. Could be more than the value of a car.)
PS: if you have any waste money, you can throw it my way!