Change it yourself, wanker, Cassidy does not say to Bernard's face. Because he doesn't want to blow this.

So he takes Wilford's $20 to the Bar and politely asks her to exchange it for British pounds and in no way indicates what it'll be used for, and anyway, she knows. She gives Cassidy the money in any case.

Which he presents to Bernard.

"Here you go, mister."

Because they don't know him at all, nope.