You have a couple issues with this:

1. Your outer do while loop requires a zero balance to exit. If they choose 'n' to exit, then you need to also zero the balance so the outer loop condition is met.
2. You are adding a negative amount, which will add that to the balance rather than subtracting it. If you are entering negative numbers as an issuance, then it would be cleaner to simply state bal += amt, and then put the outputs in their own if statement.