Two different case of determiner and adjective before individual noun

Question

The original sentence will found in second answer of this question thread.

Original sentence: 1. Es wird nächsten Sommer/im nächsten Sommer sein.

I have written the sentence below only with Preposition + Article

2. Es wird im nächsten Sommer sein.

In the 2nd sentence the determiner im = in + dem(Dativ) and adjective nächsten (accusative) are in two different case before the noun (Sommer),

So my question is: Is it appropriate to have two different case of determiner and adjective before an individual noun which they both modify?

Answer 1

Nächsten in the dative case in im nächsten Sommer, see http://germanforenglishspeakers.com/adjectives/adjective-declensions for declension tables of adjectives + nouns.