I wouldn't do it if you have any misgivings.
A good breeder doesn't 'pressure' people into buying. As a breeder I'd not let a sick puppy go until I either knew what was wrong and it wasn't an issue or I was sure the dog was healthy.
Itty bitty chihuahuas are prone to all kinds of issues including heart defects, hydrocephalus, bone break issues etc.
I'd suggest asking for a fax of paperwork from the vet showing the exam and any test results.
He may just be little - and in some breeder's kennels 'little' means normal size for a show dog.
I feel you should be able to trust your breeder and be sure they will be there if you or the pup needs any help.
If you go to
http://www.chihuahuaclubofamerica.com/
you should be able to find info on the breed club's breeder referral. What a show breeder sells as a normal sized dog the less than ethical sell as a 'teacup'.
It may be this pup you are looking at just has really poor conformation (which could lead to early arthritis onset) or it migth really not be well.
Hope this helps you!
Please press Accept if you find my answer helpful. Want more info? Just reply on your question!
Bonuses for good work are always appreciated!
Thanks for the opportunity to help you!
1 Other Expert Agrees with this!