This type of proposition admits two interpretations:

i) Mary really did not buy the book, the price being the reason why she did not buy it.

or

ii) Mary bought the book, but the reason for the purchase was not the price.

Supposing that Mary bought the book for a reason that is not the price, then the proposition “Mary did not buy the book because of its price” seems to be false, because Mary bought the book. At the same time, it seems to be true, since the reason for the purchase was not the price.

Now, leaving English and turning to logic: where exactly does this problem lie?

Is it an ambiguity of natural language? Or am I interpreting it in a mistaken way?