Personally I would rather risk losing the fridge deal then the mortgage underwriters asking more questions.
I applied for a discover card 1 month before a re-fi with a 797 credit score, very low debit to income including mortgage payment, and had to explain it to the underwriters. They didn't like it. I would believe they would have liked it even less if I applied after approval of my file.
Discover reports your inquiry same day ( in my case), and if they run your credit prior to closing it may cause a red flag.
If it changes anything on your mortgage the 500 savings on the fridge will be gone quickly.
while I appreciate (and expected) the mortgage advice, it's something I've considered carefully and discussed with all parties involved, but this post was just about Discover