"A deadline of September 30 has been set for transfer of the stake and payment of the 480 million euros by September 30," the Fininvest spokesman told Reuters.
The Serie A football club is owned by Fininvest, Berlusconi's family holding company which also controls Italian TV broadcaster Mediaset and publisher Mondadori.
The deal was signed by Fininvest and Taechaubol and approved by Berlusconi, the spokesman said.
The comments confirmed an instagram posted by the Thai businessman.
"A great honour to complete the AC Milan Agreement with President Berlusconi, a phenomenal leader," the instagram said.
Last Thursday a source told Reuters Berlusconi would sign a binding pre-accord to sell the 48 percent stake in the next few days with a final deal to be inked after the summer.
The seven-times European champions have lost some of their shine in recent years and won the last of their 18 Serie A titles in 2011.
The club has racked up debts of around 250 million euros ($274.65 million) and made a loss of 91 million euros last year.
Berlusconi, whose political fortunes have faded in recent years, has previously said that one family alone was not able to meet the challenge posed by the rising investments required to run a football club.
When the deal closes, Taechaubol will be the latest in a string of foreign businessmen who have snapped up some of Europe's most glamorous football clubs in recent years.
In 2013 a group of Indonesian investors bought Inter Milan, ending the long reign of the Moratti family over the team.
(Reporting by Stephen Jewkes and Elvira Pollina, editing by Pritha Sarkar)
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