Bus Ticketing System

I have a propective client request for a bus ticketing system. I'm having it difficult with having both an online and offline ticket purchase.

Offline you purchase from the bus terminal. So for a 18 seater bus been sold online and offline how do we streamline the sales such that tickets sold at the terminal are not bookable online and vice versa? A solution would be to have terminal attainders use the online system to book seats on behalf of customers.

But how about when there is internet downtime (especially when it rains as it is common across Africa) that customers can not book online by themselves, they would flood to the terminal and with the terminal attainder not able to use the online system, they would result to running manual paper-based sales. How do would one reconcile that and mitigate having booking records missing on the online database or a seat being booked more than once for several customers for the same time schedule?

Also how feasible is it to have terminal attainders use an offline version of the system to process ticket sales and have it uploaded to the online system at the close of the day without selling one ticket twice or more?

Will appreciate voices laid to this.