The 18-year-old was originally handed his debut for the club by former manager Antonio Conte against Saturday's third round opponents Forest.
It was also stated that the Grecians will receive further sums of £250,000 when Ampadu reaches 10, 20, 30, 40 and 50 first team appearances (substitute or starting).
The defender has made nine senior appearances for the Blues so far but has played just twice this season, with both appearances coming in the Europa League.
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