Truly the history of US coinage would have been much happier had there been no statutorily fixed connection between the value of a gold eagle and a silver dollar (letting the market set the respective values between the two). However, the reality is that Congress did define the dollar value of gold coins.
I don't think I ever suggested that they didn't. They still do. According to our government 1$ = 0.0237 oz. gold. Everything has a "dollar value," gasoline, copper, roast beef. But a dollar is not some amount of gasoline or copper or roast beef. It is some amount of silver.
And if you think this is just semantics, you might take comfort in the fact that this is what the government wants you to think. When FRN's were first issued (in 1913) they were not legal tender, but they were redeemable in gold or lawful money. As they kept changing the rules they made sure to make the changes as obscure as possible to the general public. (Pictures of predecessor FRNs here. Compare with the ones in your pocket today.) Now they are legal tender (meaning merchants/creditors must accept them) and they are not redeemable for anything.
ML/NJ