How do I adjust a transaction once they've started paying?

I get this way more than I should. Customer had like $30 worth of shit, they use their card, it says "accept partial payment of 18.91?" They hit yes, then they want to take some things off. But I cant add or remove anything once they've got that partial payment in.

I can void the entire transaction and start again, which is annoying but Ill do it. Except now that 18.91 they paid with is like on hold, it might come back in 5 seconds it might be an hour.