Hey Jason,
If you have the ability to update your JSON Structure in Mendix, and you know what the fields are meant to be,
I'd recommend manually changing them to be empty strings. So your resulting structure would look something like this:
{
"Cost of goods sold" : 100.00,
"Total" : 150.00,
"TotalForeign" : "",
"Account" : ""
}
Hope this helps,
Austin