"Would have" instead of "will have"

In the sentence "I went to the bank to ensure that I would have enough money for my trip," why is "would have" used instead of "will have"?

"Would have" seems to be conveying the subjunctive mood, but if I went to the bank already, wouldn't I use "will have" to express certainty about having enough money?

This is a simple case of backshifting, since the past tense of will is would.

Normal Present

Today he says he will do it now.

Today I am going to the bank to make sure I will have enough money for my trip.

Backshifted Past

Yesterday he said he would do it then.

Yesterday I went to the bank to make sure I would have enough money for my trip.

This is sometimes called “the future in the past”. It is frequent in reported speech.