Wa `alaykum as-salamu wa rahmatullah wa barakatuh

In the Name of Allah, the All-Merciful, the Mercy-Giving

All praise is due to Allah. Peace and blessings be upon him His Messenger Muhammad.

 

If the cost has been reduced based on this lie, then you have to correct the situation by telling your ISP the truth. You do not have to say that the other ISP told him a lie; and if he asks you, just tell him that you do not know why he said so (I suppose you really do not know why he lied to him), or just evade direct answer. A Muslim has to be truthful and honest in all his affairs; particularly in financial transactions.

 

Lying in a transaction deprives it of Allah’s blessings. The Messenger of Allah (peace and blessings be upon him) said, “Both parties of a business transaction have the right to keep or return [goods] as long as they have not parted. If both parties have spoken the truth and described [the defects and qualities of the goods], then they would be blessed in their transaction; but if they have told lies or hidden something, then the blessings of their transaction would be blotted out.”

 

Almighty Allah knows best.