A lead that qualifies for a credit can be handled in 2 ways depending upon whether you have already been billed for the lead or if you have not yet been billed (billing policy here):
if the lead was already billed to you, then we simply provide you a credit so that the next lead received into your account is free (i.e. you have already paid for it)
if the lead has not yet been billed to you, we simply un-assign it and remove it from your account and you will not see it any longer and will never be billed