The redeem underlying function converts sTokens into a specified quantity of the underlying asset, and returns them to the user. The amount of sTokens redeemed is equal to the quantity of underlying tokens received, divided by the current Exchange Rate. The amount redeemed must be less than the user's Account Liquidity and the market's available liquidity.
SErc20 / SEther
function redeemUnderlying(uint redeemAmount) returns (uint)
SEther sToken = SEther(0x3FDB...);
require(sToken.redeemUnderlying(50) == 0, "something went wrong");
const sToken = SErc20.at(0x3FDA...);