Our MAC pro purchased more than 6 years ago, we lost the invoice and now we could not retrieve firmware password. Would you please advise how we can solve it?

If you don't recall the password, then you will not be able to disable the firmware lock. If you want Apple to remove the firmware lock, then you need to provide valid proof of purchase which usually requires having a receipt showing the system serial number on it as well as your name. The EFI Firmware Password is a security feature so these are your only options unless there is a way to disable it from your iCloud/AppleID account via FindMy (I don't there is a way, but it is something to check out)?

FYI, Apple has very strict guidelines on what they will accept as valid proof of purchase. The OP needs to contact Apple to find out what qualifies as valid proof of purchase. Usually it requires the owners name and the serial number of the Mac on the receipt or invoice.

apple can easily reset it but u need to prove your bought, if you pay with your bank account u can search for the invoice or credit card u can find too, theres no way