You likely want to run it similarly to Phoenix Wright or something.

I see no problem with the "Remove Curse" bit as being an action that meant you automatically accepted the offer on your soul. As long as the Paladin was exposed to the entire deal before the Remove Curse was cast.

Do note "exposed to". Even if you didn't understand the language the deal was presented in, ignorance of the law isn't a defense (hit them with ignorantia juris non excusat or ignorantia legis neminem excusat).

Remove Curse, though being the most expedient way of dropping the weapon, isn't the only way of removing the curse. The weapon might be Disintegrated, for instance, or the wielder walks into an Antimagic Shell and drops the weapon, or one uses a carefully worded Wish, or the cursed weapon gets fed to a creature that eats magic. Just as alternate ways of doing this. Or, the wielder might hire someone to research a specific variation of Remove Curse to break that particular curse.

Note that bribery, while frowned upon in mortal matters, might be a matter of course in infernal courts. Maybe the Judge might allow some way to get out of the contract if they had something suitably nice to bribe him with - but a Paladin's soul is already a pretty good deal.

Any task assigned to be solved instead of forfeiting a soul would have to be solved fairly creatively in order to avoid the Paladin falling, if it is a 'standard' LG Paladin we're talking about.