All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

This question contains two apparent violations of Sharia:

First: It is impermissible to sell gold for postdated certified cheque as this is what constitutes forbidden usury since currency and gold share the price element that is the effective case of Riba (Usury/interest). The evidence on this is that the Messenger of Allah (PBUH) said: "Gold for gold is interest unless both hand over on the spot ; wheat for wheat is interest unless both hand over on the spot ; dates for dates is interest unless both hand over on the spot ; barley for barley is interest unless both hand over on the spot. " {Related by Bukhari & Muslim}.

Second: Selling a postdated cheque for a lesser advanced amount is another form of usury since the Messenger of Allah said: "Don't sell gold for gold unless equal in weight, nor silver for silver unless equal in weight, but you could sell gold for silver or silver for gold as you like." {Related by Bukhari & Muslim}. Similar to gold, it isn`t permissible to sell an amount of currency for a non-equivalent one. Moreover, the other bank isn`t an agent that pays to a client. Rather, it is the other way around. In fact, selling a cheque to another bank for a price lesser than its real value, as mentioned in the above question, is a form of selling debt, which is forbidden in Sharia. 

As for reaching an agreement whereby a portion of the debt is dropped in return for paying off that debt before its due time, this is a subject of considerable controversy among scholars. And Allah the Almighty Knows Best.