Final answer:
The sales account is out of balance by $135 due to the incorrect posting of a credit.
Step-by-step explanation:
When a $15 credit to sales was mistakenly posted as a $150 credit, the sales account would be out of balance by the difference between the two amounts.
To calculate this, you would subtract the correct credit amount ($15) from the incorrect credit amount ($150).
Here's the calculation: $150 (incorrect credit) - $15 (correct credit) = $135.
Therefore, the sales account is out of balance by $135.