"The US constitution specifically states that gold an silver are legal tender."
No, that's not what it says, although that is a commonly held misconception.
Article I, section 10: No state shall...coin money, emit bills of credit, make any thing but gold and silver a tender in payment of debts...
This is a restriction on the States powers, not the Federal government. State governments can't issue their own currency, and if States did choose to enforce a tender for payment of debts, the only tender they can enforce is gold and silver.
For example, a State could require you to pay your taxes in gold and silver. It can't issue it's own notes and force you to pay your taxes with them. Nor could it force you to pay debts with anything else. For example, they couldn't force you to pay your taxes with, say, $1 coins.
Basically, States have to accept any legal tender if they don't explicitly require payment in gold and silver. There are no Constitutional restrictions on what the Federal government can define and issue as legal tender.
In short, whatever the Federal government defines and issues as legal tender, must be accepted for payment of debt, with the exception that States have the option of requiring payment in gold and silver.