If you are submitting the transactions to SQL Server from a FP app, you could create a SQL Server SP to which you submit the transaction, with the SP returning the new qty as its resultset so you could apply it immediately as part of your existing synchronization. Obviously that would not work if a transaction can update more than one qty at once, or if qty in SQL Server can be updated by some other process. Also, it might not be 100% reliable if failure occurs between submitting the transaction and updating the qty in the dbf.

You could consider using a SQL Server trigger to insert a record in a "qtyforFP" table whenever a qty is updated in SQL Server. Then every 30 seconds you take a "select *... order by ttimestamp" snapshot of the qtyforFP table from FP, then scan the resultset updating the qty in the dbf and deleting the qtyforFP record. Not much can go wrong there- at worst you might update a qty in the dbf more than once if there is a failure between updating it in the dbf and deleting the SQL record.