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Insert your current balance field in your mail merge letter inside Microsoft Word

It should look like <<currentbalance>>

Now press ALT and F9 together.

It should now look like {MERGEFIELD "currentbalance"}

Now make sure this field is selected and press CTRL and F9 together

It should now look like  { {MERGEFIELD "currentbalance"} }

Change it so it looks like { = {MERGEFIELD "currentbalance"} / 2 \#$#,###.00}