I've thought about this sentence more now, and I think I might realize why I initially discarded the は for contrast explanation (before my original post on this question).
My confusion was that I felt there was no equivalency or contrasting relationship between the わたし and the お金, (the sentence topic and the sub-topic).
Thanks to your explanation, Toritoribe-san, I think I understand what's going on.
The sentence is similar in construction to our old example sentence (you helped me understand this a while back):
この車は形はいいですが、色はあまり好きじゃありません。
Here there is a sentence topic (車), and two sub-topics in contrast with each other (形 and 色). The sub-topics are not in contrast directly with the car itself.
Similarly, in:
わたしは借りたお金はすぐ返します。
The sub-topic お金 is not directly in contrast with the sentence topic わたし but instead contrasts with an
implied or
elliptical second sub-topic, in this case we will understand and supply a sub-topic like "other borrowed things". So my error was not realizing that お金 was being contrasted with another sub-topic that was understood and left out of the sentence.
Is my reasoning is correct?
Thank you!