@chrisyan I don't think you will be able to assign the value and use that as input to another label in the DM export. Instead, try using a personalization function along these lines, or else if the cheque_amount data is in the profile level you can do personalization functions to calculate the data field,
{% let cheque_amount1 = 10 %}
{% let cheque_amount2 = 20 %}
{% let Totalvalue = cheque_amount1 + cheque_amount2 %}
{{Totalvalue}}
Thanks, Sathees