If the seller did not give you the correct password, you cannot remove the firmware password yourself.
Without the proof of purchase to which Mr. Galt and the Apple Support document, Apple will not remove it for you, either. Firmware passwords were designed to provide extra security – with the understanding that should you set one, forget it, and also lack proof of purchase, you could permanently turn your own machine into a "brick." In the case of a second-hand machine with no proof of original purchase, the working assumption will be not merely that you locked yourself out, but that by locking you out, the machine is doing exactly what its owner wants it to do.
You claim that you have work on the laptop. I don't see how that is possible unless the machine was powered up when you bought It, and you never turned it off or let the battery charge run out. Because if that Mac was turned off when you bought it, it would have asked for the firmware password the very first time you turned it on. There would have been no chance to do any work on it, and therefore no chance to lose any work on it. Also no data of yours on it that you would need to wipe before returning it to the seller.
Sorry, but it sounds like you bought a "brick" that will remain a "brick". Presumably an Intel-based MacBook Pro, and very likely an older one that can't even run one of the "most recent three" versions of macOS.
Next time I would suggest buying a new, or Apple Certified Refurbished, Apple-Silicon-based Mac from Apple or from an Apple Authorized reseller. Not some unknown machine from the "back market".
