/ is a regular word, not a slang one. I know exactly what it means but am not too sure how to say it in English. "Tab" maybe? You keep an account at your bar and pay monthly or something. The word comes from the verb []{} which means "to enter (numbers in an account)".
means you (are permitted to) keep a tab.
Regarding the bribe, it is saying that the workers bribed the to have him negotiate with the bar so that the workers can keep a tab and have the company pay for it from their wages periodically.