hot to catch value with in a transaction

Dear Friend.
i need help. i m developing desktop project. where i have a invoice form. though this form data will save in 2 tables. Master record save in invoicemaster table and detail save in invoice detail table. so i used transaction but the problem is this i need invoicemasterid save in invoicedetail table also. but i cannot get this value cause of data will show in table when transaction complete. and transaction cannot complete until i get this value for detail table also
i hope my problem is understandable.
so can anybody tell me how to catch this id within a transaction
i m uploading my code file please check and if anyone have idea abt it or any better solution please tell.