Where did you make this trade? Did you make the trade with a member of this website? If so, then you can report that member to the mods. If not, was it a random trade, on Discord, or another forum? If so, perhaps you could report this person to the Discord mods or the mods on the other forum. If it was random, reporting the player is not an option because Pokémon Sword and Shield lack a report feature unfortunately. Next time, I recommend that you only trade Pokémon with trustworthy people. Nonetheless, if all you need is a Rusted Sword, I can get you one. I have extras from Zacians that I have received in trades, but never had any use for because I have my own.