In Lesson 14.7 (German Cases part 2) there is a sentence
Ich kaufe ein Buch für meinen Bruder - I buy a book for my brother.
I think this is to provide an example of how für takes the accusative.
(Different prepositions take different cases. A bit like Latin!)
Would it also be correct to use the dative, i.e.
Ich kaufe meinem Bruder ein Buch. ?
And if so, which is more likely to be used?