Cashu is NOT sats. A Cashu token (usually called ecash or "nut") is just a token, an IOU, a "voucher".

The nice thing about Cashu Protocol is that it creates a standard for some providers (mints) that are Lightning nodes to create ecash pegged 1-to-1 with sats. So if you go to a cashu mint to get some ecash the mint will ask you to pay an invoice of X sats, you pay the LN invoice and the mint returns to you X ecash tokens. The mint promises to let you redeem the tokens 1-to-1with sats.

This is how the cashu protocol works