Money is just a certificate of value, and there is a consensus for that value among a group of people
A ten-dollar bill cannot certify that something priced at ten dollars indeed has this (or any other) value, so it makes no sense to define money as "a certificate of value."
There is no standard unit of value, it is all subjective, but you can make a rough compare of things with different value: a $10 bill has 10 times larger value than a $1 bill
There is, of course, a standard unit of value: it is called money. You have just asserted that ten of those units are worth ten times more than one of them. Additionally, the price of anything you sell depends not only on your individual evaluation, but also on that of the buyer. So its value must become objective to both of you, precisely, as money.