To Get customer Transaction Balance in Axapta you can try following code to get balance for particular customer. You can try code in job to check output.
CustTrans custTrans;
;
select sum(AmountMst) from custTrans where custTrans.AccountNum == '00000343';
info(strfmt("%1",CustPaymManTrans::openAmountMST('00000343')+custTrans.AmountMST));