"firm something up" vs "firm up something"

[Tofu]

Hello Alan, Mister Micawber, Beeesneees, Mordant, Esl_Expert and other native English speakers,

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"firm something up" / "firm up something"
to make arrangements, ideas etc more definite and exact: We're hoping to firm up the deal later this month.
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Can I say "decide on" or "fix" instead of firm up?

[Mordant]

If I were to replace "firm up," I would use "finalize." "Decide" seems a bit off because a deal can be firmed up after a decision has been made. It's often used when you want to work out the last details or make it official. "Fix" could give the impression that the deal is flawed, and that may not be the case.

[Tofu]

Thank you, Mordant.