The answer is 31$ and 63 cents

Method : Lets assume Cent = z and $ = 100z    , no. of dollars =x and no.of cents = y

So,Initially he had 100zx + zy

Now after cashier’s mistake it becomes 100zy + zx

After spending 5 cents it becomes double

So now equation is 100zy + zx -5z = 200zx + 2zy

or      199x = 98y -5

Now,value of y cannot be greater than 100 (otherwise it becomes $)

so plotting values of y for 1 to 100,we get only 1 integer value for x  i.e. 31 for y = 63

Hence the answer 31$ and 63 cents

Enjoy!