You experience a problem where you void a sale but see that the value of the gift card has already gone through the till. Therefore, you can see an amount increase in your balance.
The problem most probably occurs because of a crashing machine or payment processor that causes a failure on the void operation where it should void both orders (Sale and credit). When the sale was voided, the pairing cancellation was not voided and caused the total report to be imbalanced.
In order to solve this, the pair order will need to be found and voided.
Raise a support ticket mentioning the till, the transaction id of the voided sale, and the date of the transaction, and the Prologic support team will be happy to assist you.
Use the information provided in the article Till processing crashes leaving a wrong balance to find the pairing credit order with the same amount as the voided sale and void the order to correct the report calculation.
Check the till transaction record and test your fix by checking if the till balances after you applied the fix.
Note: Whenever making changes in a database, always create a backup first in order the fix doesn't work and you need to roll back.