I work the odd time for someone I know well who runs their own business, let's call the person Joe. I would do a few hours 1-2 times a week on average probably. I know Joe very well so it's not a strictly formal arrangement, but technically I am a subcontractor.
The other day I was doing a job for Joe. The customer had asked for 2 workers, so it would be me and another guy.
I ended up being 15 minutes late to the job, the other guy being there on time. The other guy didn't start without me (even though he could have, but that's a side issue).
So Joe had to pay the other guy for the 15 minutes he was waiting for me. Joe is now demanding that I pay him that money in order to cover the payment to the other guy, because it was my fault the other guy was waiting around.
I accept full responsibility for being late and whatever way Joe wants to deal with me because of that is up to him. But is it right of him to expect me to give him money to cover the other guys wages?
I'd ask Joe to justify why the other guy didn't start the job without you.