If I leave a table, can I enter the same table with the original amount that I had bought in with?
No, a player who attempts to re-enter the same table must enter the table with the same amount of chips that he or she had when leaving the table.
For example, your original buy-in amount is $20. You win $40 and then leave the table with $60. If you decide to return to the same table, you must buy-in for the same amount that you left with - $60 in this case.
If you enter a different table, you can freely choose the buy-in amount as long as it is at least the minimum buy-in amount.