Cancel an Assignment Contract

[Jacobus Rex]

Hi, 

Can someone indicate the proper wording to have an assignment contract revoked?  I assigned a contract to a relative who wanted to buy a house on which I signed a purchase agreement. But unfortunately, he has discovered he is unable to take it. So I need to resume the contract to work on fulfilling my purchase agreement with the seller.

Maybe nothing needs be done?  The assignment verbiage indicates it is effective after assignor receives 'good and valuable consideration' i.e. a fee, but that isn't ever going to happen. 

Thanks for any constructive input.