Taker fee is underestimated due to incorrect fee calculation

Summary

The taker fee is calculated in a wrong way leading to an underestimation of the fee.

Vulnerability Detail

The taker fee is calculated by picking up the maker amount and multiplying by (1 + fee). The maker amount is the amount the user receives. Consider the following example: Assume that the ratio between the maker and taker assets is the same. The taker wants a making amount of 1000, paying a fee of 1000 * 10% = 100, so the taking amount is 1100. This means that effectively, the taker pays a fee of 100 / 1100 = 9.1%`, as they pay 1100 and receive 1000.

Same goes for the inverse calculation.

Impact

Fee is smaller than intended.

Code Snippet

https://github.com/sherlock-audit/2025-04-1inch/blob/main/limit-order-protocol/contracts/extensions/AmountGetterWithFee.sol#L33-L37

https://github.com/sherlock-audit/2025-04-1inch/blob/main/limit-order-protocol/contracts/extensions/AmountGetterWithFee.sol#L55-L60

Tool Used

Manual Review

Recommendation

The correct implementations are:

takingAmount = makerAmount / (1 - feeRate)
makerAmount = takingAmount * (1 - feeRate)