If the investor is valuing it at 5mm, so are you. You are telling the investor that the company is worth at least 5mm. If it sells for 2.5mm, you screwed up.

If the investor is valuing it at 5mm, so are you. You are telling the investor that the company is worth at least 5mm. If it sells for 2.5mm, you screwed up. Why should you participate in the proceeds of a sale?