The Changing consideration clause

In the study of individual contracts, we face agreements that, if the promise is executed before the certain time or after the deadline or promisor perform it in a particular way, the consideration will be changed. The parties determinate the consideration of promise; but they stipulate if the obligation performs in agreed circumstances, the consideration will be increased or decreased. Iran civil code isn't talked generally about the validity of such agreements and just in some cases, like article 509, has affirmed some of these clauses are valid. The lawyers only have paid attention to these cases, but they are not unanimous in analyzing these cases. Some of them have analyzed them as indeterminable consideration ,and another have proclaimed them as a warranty or a liquidated damages; so they have presented the different analysis of nature and the validity of changing consideration clause. But the attention to the common will of the parties seems, and in general, there isn’t any reason for the voidness of such clauses in the Iranian legal system.