Form Peronalization in AP Invoice Workbench - Did not receive Expected Result

In AP Invoice Work bench, the requirement is to prevent the user from entering an invoice amount higher than a Tolerence amount set. I tried a When Validate Block Trigger on the Inv_sum_Folder block by throwing a Error. But If the user directly clicks on any other tab after changing the Amount. The change gets saved even though the error gets Triggered. Any suggestions to prevent this other than Custom.pll