When selling owner finance, do you use a pre-payment penalty?

[Carole G.]

We have buyers for a couple rental properties we have.  They are each around $250k.  We are wondering how to protect ourselves in the event one or even both decide to sell or refinance in the near future, or even in the same year.  Our tax bite would be horrendous!  

We are thinking of putting in the purchase agreement (drawn up by a lawyer) a pre-payment penalty of 20% of the outstanding balance in order to hopefully cover the taxes we would have to pay.  

If we had these in a revocable trust, would it help at all?  What have you done when selling owner finance to protect in case of early repayment?