The state stipulates that whoever receives foreign exchange, who exports, who exports, will be refunded. There is no way for your client's registered Hong Kong company to get a tax refund. Only your trading company in Shenzhen can refund the tax.
If you want to get the tax refund, you must sign a contract with the factory, let the factory invoice you, and get the tax refund with the VAT invoice and customs declaration form.
Your European guest wants to keep the tax refund in his hand, and only after your trading company has refunded the tax, can he transfer the tax refund to him. His Hong Kong company can't get a tax refund.
So, it depends on how you talk to your customers. Trading companies mainly turned to tax rebates. If you can't earn a tax refund, you can only earn less, let the customer pay you, or charge him an agency fee.