Is this normal?

I took a reliever job that was advertised as ₱5,000 per 24 hours, but the “head GP”, the one with the contract with the institution, only gave me ₱4,500. Apparently, since I was just a reliever and it was originally his schedule, he kept the ₱500 for himself. He does this to other relievers as well. This didn’t sit well with me at all, so I decided not to take any more reliever jobs there.

Is this practice normal? I’m a new passer who is currently moonlighting so I’d like to ask if this is normal in the field. I didn’t expect na ganoon yung reaction ng head GP sa amin mga relievers. He is only 3 yrs ahead of us po.

edit to add: I had to cover for this GP for 6 days, and I received my salary with a 1-month delay, which I understood since it’s a public institution. Na shock lang talaga ako kasi delayed na nga yung sweldo, may kaltas pa.

edit to add more: Thank you for all your advice po. I just wanted to make sure that this really isn’t normal and that my feelings were valid, that I was taken advantage of.