Almost cheated in the clinic of Thailand

Yesterday I was in dental clinic ,having teeth-clean service ,and it was my first experience about dental stuff in Thailand ,
3 day ago ,manager and I talked over any detail,prices,etc
When I came in clinic manager told me we have do COVID test and it’ll be cost 1000 for each person ( my friend came either)
She told “It is nessesary ,otherwise doctor have a right remove an appointment..”
I was confused , we didn’t negotiate this way ,and I’m like okay,remove my appointment ,I gotta go.
When I said it-she changed her face and like okaaayyyy sorry,you can go to doctor ,okaaay
What it was?
Why Thai people sometimes wanted charging money from farang?

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