What to do if a worker plays a game of cat-and-mouse to avoid being terminated?

Q: “We have an employee who performed very poorly. Numerous staff members complained about his performance and on many occasions our administrator discussed these issues with him. We posted his position in order to find a replacement for him. In the interim he called in sick. We don’t know if this was a trick to force us to keep him. Now he’s requesting to return to work. Since he was still within the 90-day trial period as spelled out in our Personnel Policy Handbook and his position has been posted to find a replacement, are we required to take him back, at either his current position or another position?” – Mendel, New York