what is a good agreement to share in the overages

@Frederick Brown We have an area called the Fileplace. There may be a sample document there.

Tax sale overages is a strategy that many look at as a predatory practice. I recommend you consult with an attorney to do this in your area. I deal with tax sale and write my own contracts all the time. I would not create a form like this myself. I would go to an attorney. 

@Frederick Brown

Many states have limits on the amount you can charge for providing an “overage” service. Most common max is 10-15%