In the public sector it is often called “clawback”.
For example, a seller might want to share in future increases in value triggered by the grant or the implementation of planning permission, or take a percentage of future sales following the grant of planning permission.
Perhaps the seller is a farmer selling off bits of land to a developer, and the farmer is only willing to do so if he gets a piece of the action down the line when the developer has built expensive homes and started selling them off. Or maybe an initial sale of land was at a low price to reflect unquantifiable problems such as contamination or something nasty on the title. If curing the problem turns out not to be as expensive as first feared then the seller might want to take a share of increases in value or sale-on proceeds.
Sometimes it might just be “icing on the cake”, the seller will add it on to the terms if it can get away with it.
Can overage ever be future proof?
No, not really. That oft-quoted distillation of profound philosophical truth made by Donald Rumsfeld in 2002 illustrates the problem:
“There are known knowns; there are things we know we know.
We also know there are known unknowns; that is to say we know there are some things we do not know.
But there are also unknown unknowns – the ones we don't know we don't know.”
That is not to say overage is a weapon of mass destruction, although I once received an overage clause about 30 pages long which certainly had a pretty devastating impact on my mood that day (and “blew the bloody doors off” any hopes my client may have had to get the deal done quickly).
We can try and deal with all the “known unknowns” we can think of, like whether planning permission will be granted for something juicy, or whether it will be implemented, or whether the market will go through the stratosphere. These are circumstances or outcomes that are known to be possible, but we don’t know whether or not they will take place.
But what about the “unknown unknowns”, the things we haven’t conceived of when we sell the land? Or to put it another way, the things the client will probably expect you to have known but which no sentient being could realistically have predicted!
The lesson is that you cannot, unless you are a Timelord (in which case I would venture to suggest you should probably have more interesting things to be doing), draft overage agreements to cover all future possibilities.
Any practical tips (apart from investing in a TARDIS)?
Try not to make the overage period too long. The greater the length of time, the more risk there is of an “unknown unknown” materialising.
Throw in as many triggers for payment as you can think of, whilst being realistic and staying within the bounds of reason and sanity.
If it all seems too difficult, forget about it altogether and try and put the price up. Although, given the way things are at the moment, to do that you might need the gift of time travel.