Elgin claimed he had a firman that gave him permission to take the sculptures. But…
"If a firman had ever existed, it would have been presented to the British Museum at the time of acquisition to affirm the marbles’ legitimacy. Instead, what we have is an Italian document claiming to be a translation"
Zeynep Boz, head of Turkey’s department for combating the illegal trafficking of antiquities.
https://www.ekathimerini.com/culture/1255309/if-a-firman-had-ever-existed/