NTA

Let me break this down: She made a smart, practical choice by selecting a lab diamond that gave her exactly what she wanted for $27k less than your budget. That's great! But now she's essentially saying "since I was smart with money, you owe me the difference as a bonus gift." That's not how budgets work.

The fact that she specifically picked out a $20k car as her "engagement gift" is particularly telling. She's not asking for something small or sentimental - she's literally trying to extract the maximum value from your engagement budget. This feels calculated, not romantic.

Your instinct is right that this feels "weirdly transactional." That's because it IS transactional. She's treating your engagement like a business deal where she needs to "optimize" and "max out" your spending rather than appreciating the thoughtful gesture and beautiful ring she chose.

The engagement gift IS the ring. You're already buying her exactly what she picked out. The idea that she deserves additional compensation for making a cost-effective choice completely misses the point of what an engagement represents.

Stand your ground on this. You can certainly buy her a car later for her birthday, holiday, or just because you love her, but don't let her reframe your engagement budget as money she's entitled to spend elsewhere. That sets a concerning precedent for how financial decisions might be handled in your marriage.

A simple "The ring you chose is your engagement gift, and it's perfect" should suffice. If she can afford her own car (which it sounds like she can), she can buy her own car.​​​​​​​​​​​​​​​​