Let's say the merge field with the amount is called myamount and contains 0.00
One way - you put the entire paragraph inside an IF field like this:
{ IF { MERGEFIELD myamount } = 0 "" "
Your current debt is: { MERGEFIELD myamount \#$,0.00 }" }
where all the {} have to be the special field code braces you can insert using ctrl-F9 on Windows Word.
i.e. you need to press Enter immediately before the word "Your" so that you get a paragraph mark at the beginning of the text when it is present.
If you need to have different output depending on whether the amount is positive or negative, you can use the facilities of the # numeric formatting switch (they are described in Microsoft's documentation).