Return the system to the seller for a refund. 

The other option is to add the cost of a replacement system board to the purchase price.

This is not a solution.... Thank you anyway...

Unfortunately @ejn63 and @filbert are spot on. To repeat the options given to you and the reasoning. A locked BIOS system can only be unlocked with Dell tech support help. And in that process you would have to confirm and prove you are the registered owner which may be difficult in your situation. Without that information the system is basically unusable and hence the suggestion by @ejn63 that you either return the system for a refund or replace the MB. Not what you wanted to hear but it is sadly the only options.

EDIT: You can Google search for 8FC8 but it is a bit of a wild goose chase. A YouTube content author only got around it by soldering in a new BIOS chip which involves a qualified experienced tech.

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@DBSDBS wrote:

The company that sold it to me also do not knows the password.

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That is suspicious. Was that company the original owner of the computer? They should have the password if their IT people set it up.

Or, is it just a used computer dealer? That computer could be stolen.

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@DBSDBS wrote:

Now this perfect machine is unusable.

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In fact, it was unusable when that vendor sold it to you.

ejn63 gave you your options. Since the computer is BIOS locked, it cannot be unlocked without the password. Dell could help, but you would have to prove that you are the legitimate owner and, as it is out of warranty, you would have to pay for the assistance.

Otherwise, return it since the vendor sold you a locked and unusable computer. The only other option is a new motherboard.

How to Reset or Clear the BIOS Password | Dell Canada
https://www.dell.com/support/kbdoc/en-ca/000131024/how-to-clear-the-bios-password

Support for Precision 7540 | Documentation | Dell Canada
https://www.dell.com/support/home/en-ca/product-support/product/precision-15-7540-laptop/docs

Computer was bought from a second hand dealer. I have all the information about the previous owner - and NO, the computer is NOT stolen!

Now I'm in connection with DELL support, and waiting for their offer.

Best of luck with this issue working with Dell. I purchased a Precision 7760 over a year ago, continued to have BIOS problems and since then have received 3 replacements, wasting hundreds of hours and dozens of support agents including resolution team. I am now using a 7770 and I panic every time Dell puts out a BIOS update. Sorry to say this is an ongoing problem with their BIOS developers and support folks have no idea. I wish I had a suggestion for you, but after a year, I gave up on Dell providing solutions. I've purchased 4 Dell laptops since 1990s, and will never give them my money again. Good luck. 

For future reference:

An approach to creating an unforgettable UEFI / BIOS password for a Dell laptop.

Use the make and model number that's written on its case. E.g:

A Dell Precision 5540's UEFI / BIOS password could be dell_precision-5540 or simply precision5540

I must say, the Dell BIOS lock is working very good as intended.  It locked out unauthorized user and restricted access to the system as the owner's wish.  Don't know  why anyone would complaint about the BIOS lock feature when it's working so well.

It reminded me of an old saying from a Buddhist monk, he who ties the bell has to untie it himself.  In this case, Dell designed the lock on these machines, Dell has the ability to unlock the BIOS. 

If the owner forgot the password, the solution is very simple, contact Dell support, provide proof of ownership and pay for the service.  You then will receive a master passcode to unlock your machine.