Jamison has two call options on the underlying $S$ with initial price $S_0 = 24$ and strike $K=21$. One option is European and the other is American. A European option is one where you can only exercise it at expiry, while an American option can be exercised at any time.

The European option is valued at $3.21$ while the American option is valued at $3.15$. You also have access to bonds with discount rate $Z_0 = 0.9$. The underlying pays no dividends.

What is the arbitrage? Give the answer in the form of the initial credit you receive.