When exchanging RMB, you are selling foreign currency to the bank.

From the bank's perspective, they are buying foreign currency, so it should be the buying price.

In addition, if you are remitting money, the buying price should be spot exchange, not cash buying price.

The exchange rate of the spot purchase price is higher than the exchange rate of the cash purchase price, which means you can exchange more RMB.

Additional answer:

Currently, the mainland banking system cannot accept Taiwan dollars. If you go to the major bank websites to check the exchange rate, there is no exchange rate between Taiwan dollars and RMB. Therefore, remittances can only be made through international currencies such as US dollars. Regardless of the currency, exchange is based on the spot exchange purchase price.