Can someone explain why が is used here to ask a question, instead of か? If it is to be more polite, can both be used interchangeably?
Situation: A man is ordering something at a burger shop.
Conversation:
男の人: すみません、スペシャルバーガーのセットをください。
店員: はい、ポテトかサラダがつきますが。
男の人: サラダをお願いします。
This が is not a replacement of か. You cannot use か in the first place; ポテトかサラダがつきますか ("Does it come with either fries or salad?") doesn't make sense as a question from the clerk.
This が is the が which you probably remember as "but". In this sentence, が is there to provide background information, and the actual question (どちらにしますか, "Which do you like?") has been omitted.
はい、ポテトかサラダがつきますが（、どちらにしますか）。
Okay. It comes with fries or salad, so...(which do you like?)
In Japanese, it's common to omit the final part of a sentence if it can be inferred. If a sentence ends with "が?" or "けど?", something like "what do you say/think?" or "what does it matter?" is often the omitted question. Similar examples:
