Hi,

You can acomplish your goal by hooking to the igGridUpdating's editCellStarted event and in it set the max value of the AmountToPay column based on the value of cell for the AmountDue column on the same row (getting that using the igGrid's getCellValue API method).

Since you're using MVC, you need to use jQuery's live function in order to hook to the afore-mentioned event like so:

$("#grid1").live("iggridupdatingeditcellstarted", function(evt, ui) {
    if(ui.columnKey == "AmountToPay") {
        $(ui.editor).igEditor("option", "maxValue", ui.owner.grid.getCellValue(ui.rowID, "AmountDue"));
    }
});

PS: Our help article on event handling will also help you understand the logic I'm using.
PPS: I'm attaching a simple HTML page which shows this solution in action - feel free to download it and give it a go. 

Cheers,
Borislav