Well , Initially i set it to the Contract account itself, (BTW the Vendor module is being updated normally...but my issue was on the GL level - the Supplier GL account not being updated) but then when i continued the whole project and finally received a payment from the customer (and applied) then the payment offset the my initial invoice on GL.
As a conclusion , i am going to fill in the contra account with the supplier GL account, that works for me on a functional level but is it practical?