Cash Currency to pay inside Beijing International airport

I’m afraid you cannot. You need to exchange it to Chinese Yuan first, then you can buy what you want. By the way, there are following exchange counters for your reference:

T2：
The foreign exchange counters of Hang Seng Bank and Industrial & Commercial Bank of China can be found at public area on the first floor, International Arrival Luggage Hall and Restricted Area on the second floor.

T3：
T3-C Departure Hall on the fourth floor
East exit of International Luggage Claim Hall