I recently requested permission of an author to translate one of his books, and permission was granted. However, the contract stipulated that no more than 50% of the book could be translated. What is purpose of such a stipulation, and is it considered reasonable, common practice?
Pretty much meaningless. Most books can be cut a lot more than 50% to get all that is useful for NF, or needed to understand for fiction. If you got 50% then just translate the best 50%. You may only need 20% to get what you want from it.
Who did the limitation? The author or the publisher? A publisher may think they can make more money with a translation themselves at some point. The author may just be naive at how the business works.
Which contract? The original one from his publisher or the one you would sign to do the translation after they gave permission?