A: According to the Announcement of the State Taxation Administration (STA) on Issues Concerning Withholding and Payment at Source of Income Tax for Non-Resident Enterprises (STA Announcement No. 37 [2017]):

IV. If the withholding agent pays or is due to pay amounts denominated or priced in a currency other than RMB, the foreign exchange rate shall be converted as follows:
(a) For withholding agents withholding enterprise income tax, the taxable income of the non-resident enterprise shall be calculated by converting the amount into RMB using the mid-rate of the RMB exchange rate on the date the withholding obligation arises. The date of the withholding obligation is the actual payment date or the due date for payment.
(b) If a non-resident enterprise receiving income voluntarily declares and pays the tax subject to withholding at source before the competent tax authority orders payment within a specified period, the taxable income shall be calculated based on the mid-rate of the RMB exchange rate on the day preceding the issuance date of the tax payment certificate.
(c) If the competent tax authority orders the non-resident enterprise to pay the tax subject to withholding at source within a specified period, the taxable income shall be calculated using the mid-rate of the RMB exchange rate on the day before the date the tax authority issues the time-limited payment decision.

Source: STA

https://www.chinatax.gov.cn/chinatax/c102449/cjwtjd.html