Find unmarked transaction amount for customer transaction using x++ d365fo

CustTrans custTrans ;

 CustVendOpenTransManager manager            = CustVendOpenTransManager::construct(salesTable::Find("Test0054"));

 SpecTransManager        specTransManager   = manager.parmSpecTransManager();

select * from custTrans where custTrans.accountNum ='US01' && custTrans.invoice ='test123'

CustTransOpen custTransOpen = CustTransOpen::findRefId(custTrans.RecId);

AmountCur unmarkedAmount = specTransManager.findUnmarkedAmountForRef(custTransOpen.AmountCur,

                                                                     custTransOpen.company(),

                                                                     custTransOpen.TableId,

                                                                     custTransOpen.RecId);