paying a peasant to work
Peasants (serfs) were not paid. They were bound to the land they worked, and were given a fraction of the harvest they produced. The rest was property of the Lord who’s title controlled the land.
There was a (very small) artisan class where the concept of payment existed, though often it was payment-in-kind - smith the plow for my oxen and I’ll give you some food after the harvest. Money was rarely encountered for the vast majority of people.
There are plenty of things that can cause fires that are not oxygen, and don’t contain oxygen.
The halogens, Fluorine and Chlorine in particular, are powerful oxidizing agents on their own and can produce flames in the same manner as common flames.
Here’s a report on the spectra of flames produced by combustion in a Fluorine atmosphere (PDF warning).