Totally.
These are mean dishonourable people.
Cancel the job and refuse any further discussion about it.
They have zero respect for you or what you do.
They are trying to screw you.
Trying to renegotiate is the height of disrespect and is a complete breakdown of the relationship.
Decades ago as a 19 year old, a friend asked me would I babysit for the weekend for the neighbours of her regular family, in two months time.
I said sure and a rate of £100 was agreed for 3 full days, they were off to a wedding.
48 hours before the wedding the woman rang me at home and told me that they thought it was too much money and she could only pay me £70.
I was pissed off and told her I would ring her back
My parents advised me to cancel the job.
They felt it was rude and that they were trying to take advantage of me.
They said it was up to me, but what they were doing was very shabby.
I agreed. I already had a summer job, we all did, so this was extra money, not crucial.
I rang her back and said thanks but no thanks and got off the phone really quickly as she spluttered.
She rang back immediately and said I could have the full amount, which pissed me off even more.
I told her No thanks that I wouldn't feel comfortable doing it now.
She had the cheek to go back to her neighbours and bad mouth me, but my friend put the family straight and told them that she was very annoyed because I had done HER a favour.
They apologised to her for putting her in that position.
I learned a lot from that experience and can rememberit clearly from 40 years ago.
They were cheap, and thought they could exploit a teen.
My parents gave me good advice.
OP, they are untrustworthy.
Cancel and get another gig.
You have time.