I have paid all the fees, but the developer gave my master key to the property management company, and the company detained it for not paying the property management fee. Is this legal?

Strictly speaking, it is illegal, but it is reasonable. You bought a developer's house. I should give you the key after paying all the fees. But in the later period of the house, it is necessary to maintain and increase the value through the standardized management of the property. The daily service of the property needs to pay the normal management fee. For your personal reasons, you can't get the key in time. This is also an industry practice and a prerequisite for ensuring the normal work of the property. The developer and the property have a contract. There should also be a contract between the property and the owner. Simply put, the property provides order maintenance, environmental sanitation cleaning, greening, facilities and equipment maintenance, etc. And the owner must pay the management fee according to the relevant service contract provided by the property.