Hi legal brains 
I have totally forgotten the term that describes this type of agreement, please could you remind me! I need to include the correct term in a document this morning...
Scenario: a vendor is selling a property in a poor state of repair in an expensive and desirable part of London. The purchaser does not intend to make a profit from development (the building will be used for the public good, therefore the vendor is offering a good price, possibly below market value) but will not rule this out as a possibility in future, if it became a financial necessity (i.e. their service failed to maintain the required funding). Therefore, the vendor wants the purchaser to sign an agreement that says should profit be made from developing or selling the property over the next five years, then profits will be shared equally between both parties. This type of arrangement is called a....??????
TIA