[brobinson]

You don't need to trust when you can verify. The source code for the intermediary bank (smart contract) would be available for everyone to read.

[karpierz]

I'm not talking about code.

The goal of the transaction is for the Spanish bank to have access to USD. In the example given, the Spanish bank would then have to take the crypto it got and trust an exchange to give it USD in exchange for the crypto.

How do you get USD to the Spanish bank without trusting a third party?

[TimJRobinson]

There are many USD stablecoins they can use: https://www.coingecko.com/en/categories/stablecoins. The top two are the most liquid but are also centralized, many of the others are fully decentralized.

[csumtin]

USD doesn't have smart contract abilities so yes you are correct about trusting a third party to exchange crypto to USD. You could use a stablecoin but that requires you to trust the stable coin backing.