It wouldn’t be in the till if the transaction hadn’t been completed, but it is because the transaction was completed.
Therefore, at the end of the day, the till will be £100 down, but the business will be £30 down plus the trade cost of the goods sold in that £70 transaction.
I'm not arguing it is not in the till. I'm arguing it is not 'back' in the till.
Once the £100 is out of the till, it is gone. Everything that happens after, is regular business. The money and goods are accounted for, at whatever value or margin is expected. The missing £100 is not.
I can't think of simpler ways to divorce the two completely irrelevant events.