This is fuzzy, but let me try to write toward clarity.

The great problem for the nouvelle theologie , Rahner, and neo-scholasticism was to preserve the gratuity of grace. If man is created with an inbuilt orientation toward a supernatural fulfillment, then God cannot deny the supernatural fulfillment “without offending against the meaning of this creation and his very creative act” (Rahner).

But suppose God created man with an orientation toward a supernatural fulfillment (I adopt the language for the sake of argument). Apart from sin, He would invariably have granted this fulfillment simply as an expression of His free commitment to do good to creation, which He freely made. He isn’t going to scotch creation because He is not a God of arbitrary whims and windy fancies.

Is that fulfillment “exacted”? How is it any more “exacted” than creation itself? He begins the project, and sees the project through to its completion. How is the completion “owed” to the project? The creation doesn’t “exact” anything. God is faithful; what He begins He sees to the end.

It seems that gratuity of the supernatural end only needs to be “protected” if one has assumed that creation has some independent contribution to make to the supernatural completion. If all creation’s contributions are gifts, then God necessarily and inevitably bringing creation to supernatural fulfillment is simply God crowning His own gifts. This is “exacted” only in the sense that God honors God (better, the Father honors the Son and Spirit, in whom He is well-pleased). The creature has no claim at all.

Sin complicates this, of course, but the nature-supernatural debate is not about sin. It’s about creation and its finality.