It's bad for the petshop people to twist their words but it's difficult to prove since everything said is verbal.

Anyway how to prove that your maltese is a crossbred? Some purebred may look crossbred to some people just because their feature don't fit closely to the breed standard. I have similar experience with yours 15 years ago. I bought my 7 wk old Westie from a petshop and I brought him to see a vet two days later when he had diarrhea. The receptionist girl asked me what breed was my dog and when I said it's a Westie, she commented it's a crossbred Westie. The vet overheard the girl's comment and told her to shut her mouth and assured me mine was a Westie. I didn't really bother but I was surprised how she tell a crossbred from a purebred. Anyway my Westie isn't a show quality Westie and we've never asked for one when we bought him.

__________________
Grandma