So a couple months ago I found an emulator for one of my favorite Game Boy games Mario and Luigi Superstar Saga. The app I used stopped working so I cannot play it from there, but my save file exists through my Dropbox. Just a few days ago, I bought the actual cartridge (and it came in yesterday) from GameStop, and I want to continue from where I left off. Is it possible to move the save file from the emulator to the cartridge or do I have to start over?
Yes. You can do so by running the DS homebrew software GBA Backup Tool on a DS or DS Lite by using a flashcart. Despite its name, it can restore saves too. You will have to make sure your save is in the "raw" format, rather than a special emulator-specific one.