Send them a check for the 35% with "full payment of film rental for Film XYZ" written in the memo block. Include a copy of the letter from the rep. Then add a letter of your own that says, "The enclosed check is for full payment on film xyz. There has been some misunderstanding on the terms, but as you see in the attached note from the rep the agreed terms were 35%. It is extremely poor business practice to change terms unilaterally, probably illegal, so I'm sure you will accept the full payment enclosed. We look forward to playing more films from your company in the near future."
If they cash the check, they legally have no right to request more. If they don't cash it, I would be surprised.
Of course, easier said than done.