So you want to hold an open tab for the offices that they pay monthly?

Here is what I do for a similar arrangement:

Turn on Open Tickets in Square. Each time an employee from that office orders, you can save that order to the open ticket. When it's time to pay, you can just open the saved ticket and tender it out. Also, to make it even easier for the offices, you may want to consider keeping their Card on File so you can charge it at a regular time.

Hope this helps!