You can define a custom summaryType function to display your sum. According to the documentation for summaryType:


  The option can be defined as function. If defined we pass three parameters to it - the current value, the name and the record object. The function should return value. Note that this value will be used again for the group value until it changes.


For example, you could retrieve a totalSumRecord when the page is loaded - it would be a bad idea to make an AJAX call directly from the formatter - and then just display the appropriate value in the formatter:

function myCustomSummary(val, name, record) {
    return totalSumRecord[name];
}

jQuery("#grid").jqGrid({ 
  ...
  colModel : [
     {..},
     {name:'myColumn',
      index:'myColumn', 
      width:80, 
      align:"right", 
      sorttype:'number',
      formatter:'number',
      summaryType:myCustomSummary},
     ...
  ],
  ...
});



Update

There is a bug in jqGrid 4.4 where functions are not allowed as a summary type. If this affects you, here is the fix. 

jqGrid Group summary not showing total sum with paging
In Esther 7:4 we read


  וְאִלּוּ לַעֲבָדִים וְלִשְׁפָחוֹת נִמְכַּרְנוּ, הֶחֱרַשְׁתִּי--כִּי אֵין הַצָּר שֹׁוֶה, בְּנֵזֶק הַמֶּלֶךְ ...
  
  ... But if we had been sold for bondmen and bondwomen, I had held my peace, for the adversary is not worthy that the king be endamaged.


Is this true? Had, in fact, the entire Jewish population been sold as slaves Esther would not have said a word about it? Is this simply hyperbole? How do we understand this? 
