Just a year since his last extension, the Spain international has agreed fresh terms with the Bilbao-based club to bump his potential value.
Iker Muniain has extended his contract with Athletic Bilbao, signing a new deal that will keep him at San Mames until 2019.
The 22-year-old attacker agreed fresh terms only a year ago, committing himself to the Basque club until 2017 with a buyout clause of 37.5 million euros.
However, Muniain - a reported target for Manchester United in previous seasons - will now remain under contract at Athletic for a further two years, while the buyout clause has been increased to €45 million.