This could be understood simply as a contradiction, if the first speaker is saying that she will repay all egg purchases, and then later that she will not repay all egg purchases.
Alternately, this could be understood as an example of “ad hoc” revision: a generalization was stated in such a way that it appeared to be intended as always true, but then it was later modified to address a particular case. Perhaps the first speaker didn't actually expect that anyone would buy eggs, for instance, and faced with someone calling her bluff and actually buying the eggs, she backs off from it by suggesting that all along the generalization she intended to express was actually "If you get some eggs I will pay you back unless they are brand X." One wonders whether if you returned with different eggs that she didn't want to pay for, whether she might then modify the generalization to be "If you get some eggs I will pay you back unless they are brand X or from Store Y." (And perhaps she might claim that that was the actual generalization all along, but it's clear that it was not.) This kind of modification is the hallmark of "ad hoc" reasoning. It means in Latin, "for this," because a change is made in a position to address a particular case. The change is made "for this case." Often that case is an exception to the original generalization, like "I love everyone! ... oh, right, except him ... or him ..." Here it is a way of rejecting an instance of the generalization rather than an exception to it.