The buying price of cash refers to the exchange rate used by banks to buy foreign currency cash. Because banks have to bear higher costs after buying cash than buying cash. Selling cash to banks means selling foreign exchange deposits of foreign banks to banks. This foreign exchange deposit was transferred from the customer's name to the bank's name from the moment it was sold to the bank. As long as the bank does the corresponding accounting treatment, it can immediately get this foreign exchange deposit in a foreign bank and start calculating interest immediately.

If the bank buys cash, because the foreign currency cash can't circulate locally, it needs to be shipped abroad, not only can't get the deposit and interest immediately, but also have to pay the cost of keeping the cash. Only when the cash has accumulated to a sufficient amount can the bank transport these foreign currency cash abroad and deposit it in foreign banks. Banks can obtain foreign exchange deposits in foreign banks and start earning interest. The specific expenses that banks need to pay to exchange foreign currency cash include: cash management fee, transportation fee, insurance fee, packaging fee, etc. These expenses are reflected in the difference between the cash purchase price and the cash purchase price.