If I put myself in the buyer's shoes I'd feel a bit cheated. Strong word I know but the ad was misleading. Like you though Jax, I wouldn't have any need for the paper manual and your apology would be sufficient.
If this buyer is insistent on the original manual I would cancel the sale and have the amps returned. Move on to the next buyer.
If this buyer is insistent on the original manual I would cancel the sale and have the amps returned. Move on to the next buyer.