I don't think the NACHA rule book has a sample agreement. You might want to check out NACHA's webiste - they might have sample agreements in some of their publications.
If not there, then I would probably suggest getting counsel invovled as the agreement needs to cover NACHA rules, OFAC rules, UCC rules, and a whole bunch of other stuff.
A good audit program might cover some
of the things that should be in the agreement too.