If the US sanctions Russian oil, gas prices in America go up slightly, and the difference will be made up by sourcing from other countries or scaling up more shale drilling. Politically speaking, it is something American politicians will be able to risk more than German politicians can risk shutting down something like NS2, since without it and NS (which passes through Ukraine), gas cannot reach Germany, which is why Germany has vehemently opposed sanctions on NS2. Germany hasn't even bothered constructing any LNG terminals[0], something which Poland and Lithuania specifically did in fear of Russian aggression. Yes, there aren't American sanctions on Russian gas/oil yet, but that's something that's entirely able to change.

[0]: 'Plans for construction of first German LNG terminal plagued by delays and uncertainty' https://www.cleanenergywire.org/news/plans-construction-firs...