My advice: unless you're prepared to leave the bill unpaid for months and possibly take a credit hit to gain some negotiating power, take the 10% discount, pay it, and get on with life. Or, you could decide that the benefit of leaving your HSA money invested is worth the 10% and just begin monthly payments.
I went through a bunch of medical bill negotiation this year for my mother (my step father passed away). If you pay anything at all (I didn't ask the creditors what they'd accept, I just picked a number) they will happily take it and, in most cases,not send it to collections. Negotiating for reduced payments or a total write off was possible after I stopped her payments on the larger bills but if I sent in any payment at all they seemed to just keep billing for the full balance.
As I mentioned above, you might consider leaving the HSA money alone and paying monthly payments, even forgoing the 10% discount. The tax benefit and investment gains on the HSA will probably be worth more. When you've spent that money in the HSA, the tax sheltered growth on that money is gone; you can't refill the pot until next year.
I hit our HDHP's max OOP this year as my wife had shoulder surgery. I paid the bills out of pocket (using a rewards CC) and left the HSA invested.