<apex:inputField value="{!FinancialAssCont.Down_Payment__c}" styleClass="form-control downpaymentAmount" onblur="checkValidQuantity(event);" />

<apex:inputField value="{!FinancialAssCont.Initial_Date__c}" styleClass="form-control calendar_icon inital-date cal-text" onblur="copyInitialDate(this);" />

function checkValidQuantity(evt){
   if(inValidQuantity){
       showJqueryPopup();
   }

}

<apex:page>
<script>function setFocusOnLoad() {}</script>
<!-- further content -->
</apex:page>

<apex:inputField value="{!FinancialAssCont.Down_Payment__c}" styleClass="form-control downpaymentAmount" onblur="checkValidQuantity(event);" onfocus="hideCalander();" />

function hideCalander(){
     jQuery('#datePicker').css("display","none");
}

function checkValidQuantity(evt){
   if(inValidQuantity){
       showJqueryPopup();

       //Set focus to numeric text field
       jQuery('.downpaymentAmount').focus(); // This will hide the calander

      //Set the focus back to popup button
      jQuery('.ui-button').focus();

   }

}