What happens if you challenge an opponent for more points than they want to commit?

Can you fight an 1850 vs 1500 right now? What about in later rounds?

20,000 points / 15 rounds is 1333 points, assuming you lose everything (which is what I'm on pace for). Although maybe some HQs will live and I can run some all HQ or unbound HQ list at the end. This is my Chaplain+Dominus+SanPriest with a jump pack blob: 10 Independent Characters!