Since you personally made an investment of more than 250,000, your status can't be jeopardized from your partner's decision and you shouldn't worry about it.
What happens to my resident permit if the business partner who invested in the same property with me decides to quit the program?
I purchased a three-story apartment building with my business partner three years ago. We each made an investment of 400,000 euros and both got our Greek resident permits. Now he is thinking about selling his share in the property. What impact could this have on my permit? What if he sells his part not to another Golden Visa applicant, but to a local person or company? What requirements should the new owner meet in order not to jeopardize my status? What documentation should I submit to authorities after their transaction?
No effect for you, even if he sells his share.
According to the Greek immigration law, co-ownership is allowed, provided that the amount invested by each investor is a minimum 250,000. Since your investment amount is 400,000 euros, if your business partner sells his own share in the property, independently to whom, that does not affect your individual resident permit.