How should I translate "找你两块“？
To take a step back, a more complete exchange between the cashier and the customer could look something like this:  

Customer: 這包餅乾多少錢？

Cashier: 這包餅乾8元。  

Customer: 好，這裡10元。 

Cashier: 謝謝，收你10元，找你2元。 

Cashier takes $10 and gives back $2 in change.  

找錢 means to give change, specifically, change owed given the difference in amount tendered and amount owing.