Interpretation of the trading rules from the Trial Measures of Shanghai Stock Exchange for Integrated Electronic Platform Trading of Fixed Income Securities: All counterparties are dealers, including securities companies, insurance companies, fund companies and other institutions. It's just that your entrustment automatically becomes the entrustment of a brokerage firm and is considered as a "transaction between a dealer and a customer". Exchanges and registered companies will certainly not deduct this interest tax.
There are several possibilities for brokers to deduct this tax:
1. Deduct 20% interest. As for whether this 20% will help you pay taxes, it is hard to say. You can ask it for the tax payment receipt. I guess it will panic.
2. Interest tax will not be deducted.
As for what you are worried about, it is definitely impossible to deduct the interest tax in full by installment purchase, and at most, you can deduct 20% according to your installment interest.