I suppose everything is credit card based now, but I remember being in a different country years ago and having different money that didn't spend there. You had to get your money changed so that you could spend it, because the system of exchange was entirely different.
Jesus is pointing towards the difference in the Kingdom of God. The world uses a system based on rewarding those who are good to you. If that's all we do, we're no different than the world. But God's Kingdom is defined by mercy, and that mercy is generous, given to those who don't deserve it. It's a different system of relationship. It has different rules. It's not merely a transaction -- to love and serve without expectation of a reward, without it having been earned, is to reflect the love and tender mercy of God.