Significant rounding error in whitelist discount that could be avoided

Summary

Whitelisted users pay less resolver fee but the discount is applied too early leading to rounding errors.

Vulnerability Detail

The resolver fee is discounted by doing the following:

if (isWhitelisted) {
    resolverFee = resolverFee * whitelistDiscountNumerator / _BASE_1E2;
}

As can be seen, it is applied in the resolverFee itself, which just has 1e5 precision.

Impact

Code Snippet

https://github.com/sherlock-audit/2025-04-1inch/blob/main/limit-order-protocol/contracts/extensions/AmountGetterWithFee.sol#L88

Tool Used

Manual Review

Recommendation

In _getMakingAmount() it may not be possible to apply the fix directly because it is dividing by the resolverFee. Probably adding a precision multiplier would be better. For example, scaling it to 1e18 fixes the issue: Suppose the fee is 9999, taking amount is 10_000 USD, share is 50%. Scaled fee = resolver fee 1e18 / 1e5 / 2 = 9999 1e18 / 1e5 / 2 = 4.9995e16 (notice how the 0.5 is kept). Fixed total resolver fee = takingAmount resolverFee / 1e18 = 10_000e6 4.9995e16 / 1e18 = 499.95e6. The 0.05 USD component would be lost without scaling the fee. It may not be much but with enough volume it adds up.