Damn Vulnerable DeFi - Unstoppable

unstoppable3.sol

function flashLoan(uint256 borrowAmount) external nonReentrant {
    require(borrowAmount > 0, "Must borrow at least one token");

    uint256 balanceBefore = damnValuableToken.balanceOf(address(this));
    require(balanceBefore >= borrowAmount, "Not enough tokens in pool");

    // Ensured by the protocol via the `depositTokens` function
    assert(poolBalance == balanceBefore);
    
    damnValuableToken.transfer(msg.sender, borrowAmount);
    
    IReceiver(msg.sender).receiveTokens(address(damnValuableToken), borrowAmount);
    
    uint256 balanceAfter = damnValuableToken.balanceOf(address(this));
    require(balanceAfter >= balanceBefore, "Flash loan hasn't been paid back");
}