Hi, I'm a native spanish speaker, and by the way we use to talk, i can say that both of your sentences are corrects, and anyone who speaks spanish will understand what you want to say but if you're still have problems, here's a little explanation: In the first sentence: Mi abuela no quiere nada, pero LE tengo que comprar un regalo. The question is: a quién le vas a comprar el regalo? A tu abuela.. LE replaces the grandma. In the second sentences: Mi abuela no quiere nada, pero tengo que comprarLE un regalo. The question is the same, and the answer is the same too, it's just that this time you will be joining the pronoun that replaces "grandma" to the verb. Hope this will be helpful for you.

Gracias Mariano, eso es muy util.

las dos están bien. pero en una conversación yo diría "le tengo que comprar un regalo."

Gracias Victor

Both are correct.