Fees and balances

The platform fee is fixed at deployment and stored in basis points.

uint256 public platformFeeBps;

Maximum:

1000 bps = 10%

Calculation

(amount * feeBps) / 10000

For an amount of 1000 and a fee of 250 bps:

fee = 25
freelancer payment = 975

Rounding

Solidity integer division truncates fractions. A sufficiently small amount can therefore produce a fee of zero.

Balance ledger

mapping(address => mapping(address => uint256)) public userBalances;

The dimensions are:

user → token → balance

This prevents one token's balance from being conflated with another.