Employer Requesting I Send Severance Payment Back

Title. In Wisconsin, USA. It's a significant severance amount too.

I was laid off 3 weeks ago, part of the signed severance contract says I will stay on to wrap up some existing projects, which I have been doing and intend to continue doing.

The contract says the severance would be paid in two parts:

Part one would be paid "on or soon after" signing the contract. It was added to my first paycheck after signing the agreement.

Part two would be paid "on or soon after" the last date of employment (this will be in another month).

Last week, the payroll team sent me a letter requesting I send them the amount of the severance I received on my paycheck (excluding the taxes paid) as it was sent in error. They intended the entire severance payment to be made at the termination of my employment, which directly contradicts the contract.

I have confirmed with the HR team that they agree with payroll that the payment was sent in error and they insist I send it back by the end of the week. They are also citing potential increases to the payment for merit or vacation time calculations.

I'm not sure if I should continue to argue the severance was sent in according to the contract and try to keep the money or if I should get confirmation in writing of the changes and that returning the payment will not forfeit any of my benefits and I will still receive, at a minimum, the originally agreed upon severance (allowing for increases).