File sha256 and sha1 should match for the prefetch, regardless of if file is on internet or in a local folder.
Example.
April prefetch command. If you precache the file internally, the sha1 and sha256 should still be the same, and the BigFix server can use that local file if you copy it into the cache area on the BigFix server, without needing to modify the prefetch command in the action.
windows10.0-kb5012119-x64-ndp48_1bb9197b286cb2f7eba4667942abf6bd12fcf3f5.msu sha1:1bb9197b286cb2f7eba4667942abf6bd12fcf3f5 size:83996604 http://download.windowsupdate.com/c/msdownload/update/software/secu/2022/03/windows10.0-kb5012119-x64-ndp48_1bb9197b286cb2f7eba4667942abf6bd12fcf3f5.msu sha256:3e6da10a65945855e0f56346005c530114fd1c5785d2742e7e780552cad28a3d
More information about caching files on the Bigfix server manually:
https://support.hcltechsw.com/csm?id=kb_article&sysparm_article=KB0023289