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However, a fiat conversion plugin is available for BTCPay Server. Since merchants are always the owners of their private keys, they can always freely convert their coins manually, but for now there's no instant-fiat conversion. More info on the Transmuter.
You can use Transmuter to automatically sell your BTC if that is what you need. There is a guide on their GitHub page[2]. Try to set it up and if you find difficulties you can always ask here or just commission someone to do it for you. Alternatively, you can also just manually sell your BTC once per week or so. CMIIW.
[1] https://docs.btcpayserver.org/BTCPayVsOthers/#fiat
[2] https://github.com/btcpayserver/btcTransmuter