I would provide an indirect proof as follows:
Assume that you have a billion dollars and you can count your money.
For ease of computation, assume all of your money is in the form of $100 bills, the largest bill in common circulation.
Lets also assume that you can count 1 bill per second, and that you count 18hrs. per day, 7 days a week.
There are 64,800 seconds in an 18 hour period. There are 10,000,000 $100 bills in a billion dollars (assuming the American interpretation of a billion as 1,000,000,000 -- the British version has 12 zeros )
Then it takes `10,000,000 -: 64,800~~154` days to count your money. Depending on your definition of ability to count your money (is 154 days of counting 18 hours a day non-stop reasonable?), it seems reasonable to suggest that you are not able to count your money.
** If we use the British version of a billion, it would take over 432 years **
Since we have reached a "contradiction"; we assumed you could count your money, but this is unreasonable; then the assumption that you have a billion dollars must be false.
Therefore, if you can count your money then you don't have a billion dollars.