Instead the lender sells the loan, often at a deep discount ($30k for a loan with $100k balance for example) and writes off the rest.This is where a note investor comes in, they buy the loan for $30k, then go to the homeowner and restructure.

Could I take a discount from purchase price and not receive any commission? 

You are not getting a discount for that.

, but that if I don't use a broker the seller won't discount the price of the apartment (bc they don't want any other buyers in the building to think I got a better price).

Can I get a discount if I help you?""

But as @Chris Hopper said there are condos which can work even with HOA fees if they are bought discounted.

@Robert Brunson What’s the military discount at Lumber?

You can get pretty decent discounts doing it that way over a typical buyer that needs to get a loan. 

Another friend offered his 30% OFF discount voucher for Lowes & 15% OFF coupon HomeDepot (some kind of employee rewards of the month at his company).

If everything you say is accurate, I'm not sure why the seller would sell at a discount.