First, a question / comment: the formula you're using now says "if the
account balance in E2 is not zero, subtract E2 from D2". Since it's
possible- maybe not probable, but possible- for an account to
legitimately hold a zero balance, could this be the source of a logic
error? If this is totally not possible then try this formula, which is
based on your example formula:
=IF(AND(D2<0,E2<""),D2-E2,"")

If it may be possible for accounts to legitimately hold a zero balance
and you need to consider this possibility, try this formula:
=IF(AND(D2<"",E2<""),D2-E2,"")