Write down the promised adjustment
Note why the provider offered the $20, who confirmed it, the date, and where it should appear. “Credit to the next service statement” is different from “refund to the card.” A reference number or written confirmation is useful if another adult will handle the next bill.
Match the next statement line by line
Suppose the usual service charge is $55. A statement showing the $55 charge and a separate $20 credit may produce a $35 amount due if nothing else changed. A $35 total alone is less useful evidence because the plan price or other charges could have changed. Look for the actual adjustment.
Leave a missing credit open
When the next statement arrives, look for a separate credit or an adjusted balance. If it is absent, use the saved reference to follow up. Close the entry when the amount is actually applied or returned.
If the provider instead issues two $10 credits over consecutive bills, record the first $10 as applied and the second as outstanding. Do not close the entire promise after seeing the first adjustment. Confirm the remaining amount with the provider and keep the next expected statement date beside the record.