As we all know, some vendors will say anything just to get a sale. So (yet again) that's the time to get their obligations agreed (and written down) ... just before signing on the bottom line.
And regarding "dumping equipment at your door" (again, not completely unknown) ... many (thirty-plus) years ago whilst setting up a contract for a service provider ("Operations & Maintenance") I included in my forms a "Maintenance Advice Note".
This would be sent to the user (owner, customer) if we could not attend to the equipment within the usual expected period (which was a contractual obligation) ... that way we hoped to avoid penalty payments.
The form included the phrase "Please be advised that the equipment cannot be repaired immediately for the following reasons ...".
In the case mentioned the main reason would be stated as the "Unavailability of Service Manual" (etc.).
Farther down the page "The following action has been taken" would be "Service Manual requested from user" ... or if (when) failing that, "Service Manual ordered" (etc.).
Another "trick" from my contracting days would be to charge for any necessary manual as a Spare Part. This would end up itemised on the Job Report with the aim of getting the customer to pay for it. In such cases I would offer the manual purchased to the owner when the equipment was returned; but I recall that they would let us (me) hang on to it at least 50% of the time.
The bottom line of all this is to get the user to provide (pay for) the manual(s) ... especially in cases where your purchasing advice was ignored.