First of all, I don't think your main reason for breeding with your dog should be 'because you can get a puppy for free' of course.
Your dog should really add something to the chihuahua breed, so I would suggest you go to a few dog shows and show your dog to see if your dog looks good according to the chihuahua standards (size, built, etc.). You'll also have to see how the character develops (since he's only 8 months old, it will probably change a bit when he'll get older, becomes an adult) and if that character then also is according to the standards.
If the conclusion after all of this is that your chihuahua really adds something, you could consider breeding with him.
To answer your question, I don't think there are any official rules for these kinds of things, but I do know that some people indeed have 'a puppy of the litter' as their stud fee. I think it's a bit more usual though that the stud fee is the actual (retail) price of one puppy and not an actual puppy, and sometimes even the size of the litter is a decisive factor.
Even the international breeding rules according to the FCI don't state that you'd get a puppy for free; it's something that you, as an owner of the sire/stud, together with the owner of the bitch/dam agree on and write down.
'Apart from the stud fee which was agreed, the owner of the stud dog has no further rights concerning the litter towards the owner of the bitch. Specifically, he has no right to get a puppy from the litter. However, if there is a mutual agreement that the stud fee will be a puppy, it must be made in writing prior to the mating. ' (https://www.google.no/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjj8pG0z6TLAhVH7xQKHQG7DKcQFggcMAA&url=http%3A%2F%2Fwww.fci.be%2Fmedias%2FELE-REG-en-448.pdf&usg=AFQjCNELL-wVWfnkHTgJheT4nJjpU2d2Ow&sig2=jPkYFV8TCvty4V2kEdfTkQ )