I'd definitely get that in writing. If they care about the pup and know it's healthy (to the best of their knowledge) then they will not be offended. There are people out there that will find out that a pup has LS or LP or something like that and try to resell the pup so they won't have to pay to fix the problems
Go to a notary (most mailbox places have them) and make it LEGAL
If it's not notorized, they can pull the "he said, she said" thing and say they never signed it.