I often wonder how money is stored. It can't be just a number on a computer in a bank, right? Otherwise some Russian bank could just increase that number to whatever they like. And say "Look, we own 100 Trillion USD. Now let's go shopping.". So I guess USD needs to be recognized by the US somehow? Could the US simply "void" all USD that are owned by Russia?
The US sanctions (freezing their US assets) has more effect, in that regard, than a SWIFT blockade. The US seems to be the interested party in this war, and Germany is much less enthusiastic about it. Remember, SWIFT is a Belgium company and the US has been hostile to European financial institutions lately.