When I was a barman it was called Slops and it was used to conceal the fiddle that was going on either at the bar level or the cellarman level, you'd be allowed about 8 pints of ullage(waste) a night on a bar, if you can get that in the waste book but had no waste then the rest is yours. It's not "standard practise" as he claims, at least not acceptable and it shouldn't be done, it's the beer that collects in the trays under the taps and it should never be served.
Another way of dealing with it was putting it back in kegs and sending them back to Guinness as a bad keg, it's always a scam used to pump someones profit.
If he says it's standard practise the next time then ask him if they even write off waste.
I never used to do it because I considered it bad practise, unless I had a real arse who deserved it because they clicked their fingers or whistled for a barman, then it was acceptable.