If these are native FileMaker fields, you can set the balance field definition to Evaluate the comparison of the other 2 fields any time either of them is edited.
Lokk into the Auto-enter by calculation with the Evaluate function.
If I understand your equestion correctly, you could also define a calculated field in your data table which holds the result.
This would create an extra permanent field which holds that result.
I didn't understand the question either.