A transaction exchange is an exchange between two parties. What they use is up to them to determine. There's no obligation now for a retailer to accept money in a transaction unless they agree to do that.
That exists before and after the removal of legal tender status. Basically what we're saying is that if that retailer who accepted that money that had legal tender status removed came to the government and tried to say, pay their tax bill, it would not be accepted at that point. Between parties, there's always an agreement on what they're going to use in order to settle a transaction.