Teóricamente es "le". Pero cuando "le + lo/ le + la"...cambiamos a "se". Siempre Nunca dos pronombres con "l" juntos * Le lo = se lo *Le los = se los (Give It him) ****dalelo = dáselo

Look at these examples. Devolver algo a alguien A mí: tienes que devolvérmelo. You have to give it back to me ¿Me lo devuelves?, por favor. Will you give it back to me? A ti: tengo que devolvėrtelo. I have to give it back to you ¿Te lo devuelvo ahora? Shall I give it back to you now? A él, a ella: tengo que devolvérselo. ¿Se lo devuelvo? A nosotros, a nosotras: tenemos que devolvérnoslo. ¿Nos lo devolvemos? A vosotros, a vosotras: tenéis que devolvéroslo. ¿Os lo devolvéis? A ellos, a ellas: tenéis que devolvérselo. ¿Se lo de volvéis?

In Spanish, when you have double object pronouns (a direct and indirect one used together), the indirect object pronoun "le" becomes "se": Le digo el cuento a María. I tell the story to María. Se lo digo. I tell it to her. Mi abuelo les compró los regalos a los niños. My grandfather bought gifts for the children. Mi abuelo se los compró. My grandfather bought them for them. That's just how the grammar rule works in Spanish. Probably because "le lo" together sounds really awkward and difficult to pronounce.

When the Indirect Object pronoun ''él, ella, usted'' (le) is followed by a Direct Object pronoun, the Indirect Object pronoun “le” becomes “se”. You should also take in count the plural form, when the Indirect Object pronoun ''ellos, ellas, ustedes'' (les) is followed by a Direct Object pronoun, the Indirect Object pronoun “les” becomes “se”. In your example: Le has devuelto el bolígrafo a Manuel? The Indirect Object (IO) is "Manuel" and the Direct Object (DO) is "el bolígrafo" For the answer the Indirect Object (IO) pronoun come first: "Manuel" -> le (but because it will be followed by a Direct Object (DO) it will turn to "se"). The Direct Object (DO) pronoun come second: "el bolígrafo" -> lo Finally the answer will be: Sí, se (IO) lo (DO) he devuelto. Hope I helped you... If you are looking for a Spanish teacher, visit my profile: https://www.italki.com/spanish

What others have said. Spanish grammar does not allow using 2 third-person object pronouns that start with the letter "l" as objects of the same verb. Whenever a verb has 2 third-person object pronouns, one of them is an indirect object and it is always "se".