Я предпочита́ю зелёный чай чёрному.
In this sentence зелёный [nominative] should be зелёного [accusative] because чай is the direct object and therefore in accusative.
I came across this sentence and want to know if they made a mistake or if there is a grammatical reason which I am not aware of?
This sentence is grammatically sound. The reason for the apparent disagreement is the following.
The verb предпочитать takes two objects:
For inanimate masculine gender objects, the accusative coincides with the the nominative (зелёный чай) and not genitive (зелёного чая) as it does for animate objects.
The sentence employs an ellipsis that omits the word [чаю] after чёрному like in the English example: John likes black tea and Mary likes green [tea].
Hope this helps clear up the confusion.
