If an employee resigns voluntarily, you should ask them to document that fact in writing with a simple resignation letter.
If you’re terminating an employee, whether as part of a layoff, for cause, or simply because the relationship was no longer working out, you should attempt to get the departing employee to sign a separation agreement.
Typically, such an agreement is signed in exchange for financial consideration for the departing employee in the form of severance payments.
There are many different approaches to this type of agreement, and there are additional complexities based on whether an individual belongs to a protected class. You will definitely want a lawyer to carefully review any such agreement, especially if the termination may be contentious. Good legal counsel can also offer suggestions on the best way to secure a signature — including what rules may exist requiring the employee to be able to revoke their signature after the fact.
This sample was designed for a relatively straightforward termination, without a lot of special considerations.