After his wedding expenditure goes out of budget, a miser accountant decides to recover his expenses by targeting an illiterate-rich client, collecting fees in the name of various tax-saving expenses.

but there's no such thing as a negative protagonist

Although your second version is better,
I agree with Nir. It's still un-cinematic.

"When his rich illiterate client shows up with uninvited guests, the miser accountant targets him to recover his wedding expenses." Is this appropriate?

The main character won't be just filling out forms and filing taxes. He will have to work out a plan to logically charge him with his fees. Also his actions can invite troubles to his client. Eg. govt. attacking the client with scrutiny. He can charge fees to get him out of such scrutiny.

This can work as a premise for a comedy, but the wording needs to change.

I'd also add that the inciting incident isn't strong on account of it being a matter of his own doing - he let the wedding go over budget. The best inciting incidents happen out of the MC's control and dramatically affect them.

What if the mother in law made the wedding go over budget? This would at least mean the event caught him by surprise.

The other problem with this concept is that the main action is the MC filling out forms and filing tax reports... this is uncinematic, is there anything else he can do to his rich client?