What grammar rules are being employed here?

Question

I was talking to my wife (for whom English is not a first language); we had just returned from shopping, and had bought an item that I would have felt "awkward" buying by myself.

She said something along the lines of:

"If we had not bought this item today, you will have to buy it when you come back after your trip to Europe"

I corrected her and said, she should have instead said:

"If we had not bought this item today, you would have had to buy it when you returned from your trip to Europe"

She did not understand why she had to use past tense (would have had to buy ...), when we were talking about the future.

I had no answer, and was similarly surprised when I came to analyse what I was saying. What is the grammatical rule at work here?

As an aside, it appeared that the rule took the form of:

IF (SOME EVENT E DOESN'T HAPPEN):
    THEN USE PAST TENSE WHEN DESCRIBING WHAT HAPPENS IN ABSENCE OF EVENT E

For example:

If you had not inoculated your child against that disease, you would have had to spend a lot of time worrying about your child catching the disease...

Answer 1

If we had not bought this item today,

is a conditional clause (see type 3 conditional) in the past perfect tense.

you would have had to buy it when you returned from your trip to Europe.

is the main clause and has to use the matching participle; in this case, because your conditional clause was past perfect, you have to use the past participle "would have".

Because the conditional clause denotes an irrealis mood (in other words, a situation that has not happened and is no longer possible: not buying the item) you can't say

you will have to buy it...

because that doesn't match the tense from the conditional and also correlates with a possible action, which doesn't fit with a conditional that's no longer possible.

There's a pretty helpful guide on forming conditionals in various tenses here as well.