Nar-mayas and the caravan master

You should decline the offer, as the caravan master is lying somewhere.
Indeed, there are as many mothers as calves (because each mother had one calf, and the caravan master said they were only sold together).
For each pair "mother + calf", there are two options if we exclude nar-maya:

• The mother is a camel, the calf is a camel $\rightarrow$ that makes $2+2=4$ humps.
• The mother is a dromedary, the calf is a dromedary $\rightarrow$ that makes $1+1=2$ humps.

So, if there are no nar-maya, there should be an even number of humps. But $13$ is not even!

So either the caravan master is lying about there not being nar-maya, or about never selling a calf without its mother. So you cannot safely take his offer!