Device trade-in gift card going to waste!!

I traded in an old iPad for an Apple Gift Card of $280 value. An Apple team member was able to use that gift card in person to help me purchase an Apple Pencil. The leftover money from my gift card ended up as a credit on my iTunes account for $150 instead of just staying on the gift card for future use. I have not used any of its value in iTunes and I want to use it in the Apple Store to put towards my new MacBook. Everyone I’ve spoke to says this cannot be done and I’m just not sure why. It’s unused money from my own trade in, why can some be used in the Apple store but the rest is basically thrown away if you don’t use it all in a single transaction!? Has anyone else run into this issue? I’m not sure what to do….