Refund request for technical service

Hi!

Due to technical problems in the serviceman's system, a charge of €70 was attached to us when connecting our internet. We were informed beforehand that the service would be free, but due to problems with the connection, the service technician had to accept the charge in the system (he could not change it to 0). However, he indicated in the invoice that we were therefore due a refund on the next invoice (€70), which did not happen. Because of that I'm asking here for help with the refund. In the attachment I'm sending the screenshot of the invoice.

Best,
Jakub