# Logical analysis of the following situation?

I ask you to go to the store and buy some eggs. If you get some eggs, I will pay you back. You bring the eggs back. But I tell you "oh they aren't of brand `X`, so I won't pay you back".

How can you describe a situation where someone withholds information like this, whether knowing or unknowing?

• It's not about withholding information. Eggs of any kind are eggs, and the condition was that if you buy some, of whatever kind, then you will be paid. What's said at the end doesn't matter; either the promise will be broken or you will get paid. – Hunan Rostomyan May 19 '14 at 17:11
• I see. So the statement is actually about the promise, and not about the eggs at all. – user6786 May 19 '14 at 19:00
• That's just my opinion. Other, more interesting alternatives have been offered below. – Hunan Rostomyan May 19 '14 at 20:20

If this were an argument, and not a request, we would call it moving the goalposts, which is where the original conditions are met, but additional ones are subsequently added.

• Right, IF it were an argument. But there is a logical problem here that has to do with just the asserted statements, without treating the situation as analogous to argumentation. – ChristopherE May 21 '14 at 14:44

I think we cannot call it a Logical fallacy.

It look like :

Denying the antecedent – the consequent in an indicative conditional is claimed to be false because the antecedent is false; if A, then B; not A, therefore not B

with A := "you buy some eggs for me" and B := "I will pay you back"

but in addition we have an "hidden" premise C := "eggs must be of brand X".

In any case, from

( A ∧ C ) → B and ( A ∧ ¬C )