For hundreds of years, Bookkeepers have solved this problem by putting debits in one column and credits in another with the balance computed and recorded in a third. You might choose to do the same In FileMaker:
See this demo file: https://dl.dropboxusercontent.com/u/78737945/AccountingLedger.fmp12
If you choose to keep both debits and credits in the same field, you can add a calculation field:
If ( Description = "Credit" ; Amount * -1 )
You can use this field in place of amount to show the positive and negative values and a summary field can total up this calculation field to compute a balance.