[SalesForce] Visualforce – Apex Parameters are not getting passed to controller

VF Code

<apex:pageBlock title="New Expense" rendered="{!isNewExpense}" id="pb">
<apex:pageBlockButtons >
<apex:commandButton value="Save New Expense" action="{!save}" rerender="pb" >
<apex:param name="Exp_Date" value="{!Expense__c.Date__c}" assignTo="{!Exp_Date}"/>
<apex:param name="Exp_Type" value="{!Expense__c.Type__c}" assignTo="{!Exp_Type}"/>
<apex:param name="Exp_Amount" value="{!Expense__c.Amount__c}" assignTo="{!Exp_Amount}"/>
<apex:param name="Exp_Comments" value="{!Expense__c.Comments__c}" assignTo="{!Exp_Comments}"/>
</apex:commandButton>
<apex:commandButton action="{!cancel}" value="Cancel"/>
</apex:pageBlockButtons>

Controller code

public with sharing class ExpenseController
{

private ApexPages.StandardSetController sc;
private ApexPages.StandardController sc1;
public Integer SumOfAllExpenses {get; set;}
public Boolean isTwenty {get; set;}
public String pwd{get;set;}
public Boolean isNewExpense {get;set;}

public Date Exp_Date{get;set;}
public String Exp_Type{get;set;}
public Integer Exp_Amount{get;set;}
public String Exp_Comments{get;set;}


public ExpenseController(ApexPages.StandardSetController sc)
{
this.sc = sc;
SumOfAllExpenses = displayTotal();
isTwenty = getTwenty();
 
}


public Integer displayTotal()
{

List<AggregateResult> i = [SELECT SUM(Amount__c) FROM Expense__c];
SumOfAllExpenses = Integer.valueOf(i[0].get('expr0'));
return SumOfAllExpenses;
}

public void Custom_Delete()
{
System.debug('Checking for Pwd..');
List<Expense__c> l = [SELECT Id FROM Expense__c WHERE IsDeletedFromUI__c = true];
System.debug('Records to be deleted :'+l);
if(l.size() > 0)
{
if(pwd == 'KVN')
{
System.debug('Custom_Delete Invoked');

delete l;

System.debug('Custom_Delete Completed');
}
if(pwd != 'KVN' || pwd=='')
{
for(Expense__c ll : [SELECT Id FROM Expense__c WHERE IsDeletedFromUI__c = true])
{
ll.isDeletedFromUI__c = false;
update ll;
}


ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.ERROR,'Incorrect Password.Delete Cannot Be Processed');
ApexPages.addMessage(myMsg);
}
}
}

public Boolean getTwenty()
{

Integer cnt = [SELECT COUNT() FROM Expense__c];
System.debug('Cnt variable is :'+cnt);
//Integer Cnt = Integer.valueOf(j[0].get('expr0'));
if(cnt > 20) 
{
return true;
}
else
{
return false;
}
}


public PageReference save()
{
System.debug('Before Save - Comments Value :'+(Expense__c)sc.getRecord());
cancelNew();
System.debug('OUTSIDE INSERT'+Exp_Date+' '+Exp_Amount);   //*****LINE 0******

if(Exp_Date != null && Exp_Amount != null)
{
System.debug('WITHIN INSERT');
Expense__c e1 = new Expense__c();
e1.Date__c = Exp_Date;
e1.Amount__c = Exp_Amount;
e1.Type__c = Exp_Type;
e1.Comments__c = Exp_Comments;
insert e1;
}
sc.save();
Custom_Delete();
System.debug('After Save - Comments Value :'+(Expense__c)sc.getRecord());
displayTotal();
System.debug('After Total - Comments Value :'+(Expense__c)sc.getRecord());
PageReference p = new PageReference('/apex/addExpenses');
return p;

}

public PageReference cancel()
{
cancelNew();
sc.cancel();
displayTotal();
PageReference p = new PageReference('/apex/addExpenses');
return p;

}

public void needNew()
{
isNewExpense = true;
}

public void cancelNew()
{
isNewExpense = false;
}



}


}

My understanding so far :

a) User presses "Save" upon which save() in the controller extension class will be called.

b) I am also sending <apex:param>s to the corresponding properties in the controller.

c) I expect that at LINE 0, apex parameters from VF page (Exp_Date and Exp_Amount) will be sent but I am getting only null values as shown by the debug log.

06:36:07.106 (106966000)|USER_DEBUG|[85]|DEBUG|OUTSIDE INSERTnull 0

Can anyone let me know as to why my apex parameters are not sent to the controller properties ?

UPDATE :

Looks like there is an issue with

<apex:param name="Exp_Date" value="{!Expense__c.Date__c}" assignTo="{!Exp_Date}"/>
<apex:param name="Exp_Type" value="{!Expense__c.Type__c}" assignTo="{!Exp_Type}"/>
<apex:param name="Exp_Amount" value="{!Expense__c.Amount__c}" assignTo="{!Exp_Amount}"/>
<apex:param name="Exp_Comments" value="{!Expense__c.Comments__c}" assignTo="{!Exp_Comments}"/>

Because when I hard code values like "xyz" instead of "{!Expense__c.Comments__c}" in the apex:param tag I can obtain "xyz" from the controller (as revealed from the log).

Is there anything wrong in the apex:param tag especially with "value" attribute assignment ?
Can someone let me know ?