Duuude....come on. What a waste of Foie. Put it on a burger? Seriously?
Listen, if you are using wagyu meat, what is the point of putting foie on the burger? You are just adding rich food upon rich upon rich. The wagyu by itself will be dripping with delicious beef fat, why add more? Why not buy a tin of beluga and put it on there too?
Grinding it into the burger itself is a heinous idea, no offense. If anything, make a torchon of the foie and use it as a stuffing for the patty. Slicing it thin and searing it on the griddle will likely cause about 80% of it to melt away.
If you portion it and freeze it, it will oxidize. And, like butter (cause it is so high in fat content) extended storage in the freezer and/or fridge can cause it to pick up "off" flavors. So if you don't sell a lot of it, you might have a problem. If you have access to a vacuum sealer, then it is your best friend if you must freeze it.
Listen, Daniel Bolud can get away with it because he has an established, upscale clientele, established reputation, is one of the finest chefs in the country, and can charge like 50 dollars for it. But anyways, if you insist upon going through with this, then I suggest soaking it in cold water or milk overnight, curing it, pushing it through a tamis, making it into a torchon, chilling it, then slicing it, and using it to stuff inside your beef patties. This way, you can gently warm the foie in the middle of beef (say, with a medium burger), without overcooking it/rendering out all the fat. If someone wants a well done burger, say no (lol). You will also likely get a better yield from the foie this way.
But, a thousand times no to slicing it thin and searing it on a griddle. Utterly pointless. Also, please don't ever use truffle oil. Ever. Absolutely foul (my opinion of course).
I'd love to no how this turns out--please follow up. Count me as skeptical but intrigued.