Why Is a Risk-Free Profit Guaranteed in a Successful Arbitrage Trade?

A risk-free profit is guaranteed because the arbitrageur simultaneously enters into two or more offsetting transactions. These transactions lock in a known price differential at the time of execution.

Since the profit is determined immediately and the positions are closed out at expiration (or earlier) at a pre-determined or convergent price, the profit is not subject to future market price fluctuations. The only remaining risks are counterparty and execution risks, which are non-market risks.