は is used for Objective 格助詞

Question

In the following sentence, why 'は’, not 'を’、is used as 格助詞？？

私は借りたお金はすぐ返します。

Answer 1

In the following sentence, why 'は’, not 'を’、is used as 格助詞？？

私は借りたお金はすぐ返します。

No, the は is not a 格助詞. I say it's indicating the second topic/subject of the predicate, すぐ返します.

Answer 2

The particle は can be combined with numerous other particles. For example, you may have already seen things like

で＋は＝では
と＋は＝とは

However, in certain situations, the combination gets reduced as in

を＋は＝は

Similarly

が＋は＝は

These are the only two cases I know of where the joined particle gets fully absorbed into は.

So the speaker wanted to say something like

私は借りたお金をはすぐ返します

But, in modern Japanese this isn't grammatical and the をは becomes just plain は.

In older documents (or documents using an older style of writing) をは isn't necessarily reduced in this manner but becomes をば instead. And, occasionally this pops up in some dialects.