1) SUB-B is providing agency services outside Hong Kong
1.1. Hong Kong tax rules do not impose withholding tax on payments to non-residents in connection with service charge. The management fee is only deductible if the provision of management services is incurred for the production of Hong Kong source income. See section 16 of the Inland Revenue Ordinance. Where the activities from which the profits arise take place outside Hong Kong, the profits are sourced from outside Hong Kong.
1.2. However, if the management services are provided by employees in Hong Kong, then the recipient of the management fee will be liable to HK profits tax. Whether the provision of management services attracts Hong Kong tax liabilities is independent of the fact that Sub-B is earning non-Hong Kong source profits. The key point is that one looks to see what the taxpayer has done to earn the profit in question and where he has done it. See Commissioner of Inland Revenue v. Hang Seng Bank Ltd. 
1.3. You should note whether the management is an arm's length amount. The management fee can be challenged by the Inland Revenue Department if the amount is not comparable with the management fee that would have been charged between independent parties under Part 8AA of the IRO (Transfer pricing rules), which came into operation as from 1st April 2018.
2) SUB-A is selling properties located in China
If Sub-A's activities for the sale of PRC property are performed outside Hong Kong, SUB-A is earning non-HK source profits and not liable to HK tax. The management fee is deductible from or charged to taxable income or profits, without any HK tax implications. Note that if the management services are provided in HK, the recipient who delivers the services is liable to Hong Kong tax.