This letter lays out the conditions under which the employee pledges to resign at some future date if agreed-upon goals or metrics are not met.
Dear Mr./Ms. Bossname,
After reflecting on our conversation this week, I have decided to offer a conditional letter of resignation.
I am extremely confident that I will be able to meet the metrics you laid out. If I am unable to do so by the end of the time period we discussed, I shall resign.
I hope that you can place the same trust in me that I have in myself. I am truly dedicated to helping the company meet and exceed its goals.