The Spanish rehusar — literally, “refuse” — sounds odd to English ears: it’s the same word, but the ‑f- became an ‑h-. Huh?
This is explained via the pattern of Latin words that began with an f- tended to turn into an h- in Spanish and only in Spanish. See famine/hambre, and huir/fugitive for example.
Refuse and Rehusar follow the same pattern. Both come from the Latin refundere — from which we also get the English, refund. They are all ways of giving back.
This f‑to‑h pattern usually happens with the first letter of the word. But here it is the first letter of the second syllable — because the re- is of course the standard prefix so it didn’t effect the sound pattern change.