This question should be in the law section rather than here, however, I would point out a number of relatively simple solutions to this question.
First, a Euro is not money, it is a commodity. Commodities law covers contracts like these right now. Gold and corn are similarly covered. A euro is a thing.
There is clearly a common law contract present here, whether it conforms to state law is another matter altogether. It may be valid in Louisiana too if certain things were done that are not present in the example, as it is a civil law jurisdiction.
Nothing about legal tender law says that there is a requirement of contracts to be denominated in money. It is perfectly legal for you to sell me ten elephants today in exchange for an acre of land to be delivered in one month. Delivery of anything else is a breach of contract. Not only are you breaching it if you offer money, in lieu of land, but also if you offer me alternate land or eleven elephants, subject to the laws of the state the contract was made in.
With that said, federal law allows the payment of debts, explicitly, in things other than money under certain statutory exceptions. For example, if, during the refinance of a primary residence by another lender than the original, a right to rescind the contract is not offered at the end of three days, then the borrower may repay the loan without interest for up to three years. The law says the borrower is not required to pay in money if it would be unfair to the borrower.
Now, let us imagine that the contract for euros was breached, a judge is going to require the payment of damages in dollars. It may also require the payment of court costs, attorney fees, and any expenses the original party had in getting money converted to euros using their own money.
It would also likely matter, in many jurisdictions, as to whether the use of dollars or euros was material to the contract by the counterparty. For example, if the lender was going to use the money as payroll in Germany, then the euros matter. If they were just going to convert it back to dollars later, it might not matter.
The legal tender cases the courts have heard have focused on transactions in dollars. The original one happened when a contract to deliver cotton in exchange for roughly $5100 was made. The seller wanted delivery in gold coins and not just any dollars. The court held that the seller had to accept paper money, even though greenbacks were not backed in gold.
Barter exchanges that involve no money at all are rather common, even today, in agricultural areas. It is not uncommon in agricultural areas in America to allow the use of land as a leasehold in exchange for the use of some farm equipment for a fixed amount of time also in an equipment leasehold.
It is important to understand how money would not work as an alternative here. If I give you my land in exchange for the use of a combine over a fixed date, then offering me the cash rental value of my land may result in the loss of crops on hundreds or thousands of acres. The value to the user of the equipment may vastly outweigh its market rental value or the value of the land. Common law does not require any form of parity of value, though civil law can.
If you sell me a van Gogh for $10, not understanding its value, then that is a valid common law contract. It is not a valid civil law contract, though. You would have a problem in Louisiana.
A euro is not money unless you are standing in the European Union.
The original legal tender law required foreign currency to be treated as U.S. currency for purposes of fulfilling contracts denominated in dollars. That is how the idea of legal tender originated. It wasn't to require people to accept U.S.-produced money. It was to require people to accept foreign currency as if U.S. currency because of the inability of the government to produce enough of its own.
Indeed, Mexican currency was quite often the primary currency in areas of the U.S. many decades after legal tender law changed in 1857, to not accept the use of foreign currency for the payment of U.S. taxes.
The original purpose of the law was not to create an alternative to the performance of a contract but to require that any contract denominated in dollars could be paid in the money of other nations at specific ratios. It also made the Treasury accept foreign currency in the payment of taxes.
Once the government had matured to the point it could provide a stable currency, it banned foreign money for the payment of taxes or debts denominated in dollars. Instead, the foreign money could be turned in to the Treasury to be melted down for a fee or exchanged in the money markets.
Basically, what legal tender implies is that if you are owed $1,000 for a debt and someone brings you $1000 in U.S. money, then you must accept it. If you argue that the legal value of one ounce of gold is $20 and you want 50 gold coins of twenty dollars each, the court will not support your demand.
Also note that if your contract would require you to deliver $1,000 in twenty-dollar bills, then you would be required to deliver them in twenty-dollar bills. You can agree to more binding conditions if both parties choose to. In that case, delivering ten $100 bills is a breach of contract, though I suspect that unless there were real, measurable damages, that a court would reject the claim. An example of such a case might happen in a vending machine company where they need $1000 in quarters. It would be a breach to deliver hundred dollar bills and if it prevented a day's worth of sales, then you would be liable for the losses.
