Both are appropriate, and the difference is very subtle - in many instances, borderline nonexistent.
There are differences between "will" and "going to," as you know, and this example, on initial inspection, seems to be a case for "will," because the decision is made in the moment - i.e., it's very close to the following scenario:
"Can you do this for me?"
"Sure, I'll do it."
Note that here, "I'll" is far more idiomatic and appropriate than "I'm going to," as "going to" can be misunderstood as suggesting that you were going to do it anyway.
But in the dialogue you provided, the statement isn't one of instant agreement. It's more in the nature of a counteroffer. Idiomatically both versions are common. "Going to," in my opinion, sounds a hair more definitive in the sense that the person making the offer doesn't expect the other person to do anything other than accept. "Will" sounds perhaps just a touch less confident in this regard, but that doesn't mean there's anything wrong with it, and indeed you might prefer it if you don't want to sound as if you're presuming that the other person will accept the offer.