I purchased a mac brand new after update it tells me it has a mdm

Only the people who set MDM can reset the device unless you also have the original receipt for the sale of the device from Apple to the corporation. A bill of sale from a reseller is -to my understanding- not going to be sufficient.

Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online* at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide many details in the article as to what is acceptable proof. Prior to this official service Apple typically required the original receipt** issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the first owner who does this, or a descendant with legal proof of ownership transfer). Note that while this will let you get use of the device again, it will not help you with lost access to an account.

* There is a special link for this service. To save time you should use it instead of https://getsupport.apple.com/

**Apple recommends you keep your receipt after you buy an Apple device. --> Keep your receipt after you buy an Apple device - Apple Support

In future, make sure this has been done What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

The fact that it came up with a device management screen confirms that you were not sold a new, unused Mac.

Either the reseller lied, or the person who returned it lied to the reseller about its condition. No matter which is true, the Mac was not new and the person you bought it from had to have known at least that.

Receipts can be written to say anything; by accident or by intention.

If you have notices of MDM, somebody set that up before you got the Mac. That someone may have had a new Mac at the time, but when your took possession of it, it was perviously used.

Your only recourse is between you and the seller.

The only person who can remove that lock and not affect the rest of the Mac would be the person who set the lock.

I don't know if you can even buy an officially refurbished computer from anybody but Apple itself. Sure, plenty of places will sell a used Mac, but my understanding is Apple will not not unlock one unless you also provide the original sales receipt when the first owner bought it from Apple.

You should take it back to the store from which you purchased it and insist upon a refund. This isn't a warranty issue. They did not sell you a fully functional computer in the first place. Frankly, for all you know it could have been stolen.

Does your receipt have the system serial number listed on it? Does it match the serial number found on the bottom of your laptop and within "About This Mac" (found by clicking the Apple menu)? Just want to make sure the serial number is correct in all three locations.

Is the seller an Apple authorized reseller? There is a difference between an authorized reseller and a third party vendor selling Apple products. An authorized reseller has a direct business arrangement with Apple which means any mistakes would have consequences for the authorized reseller.

Anyone can acquire new unopened Macs and sell them as "new" when technically they are still pre-owned even if they have never been opened. I have seen several posts on these forums where someone bought a "new" Mac in a sealed unopened box and found out their warranty was half over because the warranty only applies from the date of the first sale which had been to the original purchaser who then resold the Mac as "new". The current owner in that situation was technically the second owner.