说 and 兑 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.说 /*l̥ot-s/, /*l̥ot/, /*lot/ 兑 /*lˤot-s/, /*lˤot-s/
speak, say, talk; scold, upbraid; to speak; to say; to explain; to scold; to tell off; a theory (typically the last character in a compound, as in 日心說|日心说 heliocentric theory)
