I applied it to my case like this:
DSum(" [Montant_debit]","OPERATIONS","N_Operation<=" & [N_Operation]& " AND N_de_compte='" & [N_de_compte] & "'") AS DEB, DSum(" [Montant_credit]","OPERATIONS","N_Operation<=" & [N_Operation]& " AND N_de_compte='" & [N_de_compte] & "'") AS CRED, [CRED]-[DEB]...