Updates with Main and Sub Forms

[Kevin]

I am new to Access and Access programming, so a simple explanation of the
following would be much appreciated.
I need to track bank transactions, some of which will be split (eg a single
payment in may constitute a number of separate checks from different people).
I thus have a table for the main transaction (and when it does not need to
be split that is the only record). I then have a separate table for the
subsidiary transactions that links to the main table.
I am trying to design the forms to update these tables. The main form
includes an entry for the amount or the overall transaction, and a checkbox
to indicate if subsidiary transactions are required. When the checkbox is
checked and subform displays that allows entry of the subsidiary transactions.
My problem is that I don't want the either the main transaction, or any of
the subsidiary transactions to be posted until a check that the sum of the
values of the subsidiary transactions equals the value of the overall
transaction has been satisfied.
At present the subsidiary transactions update as soon as I move to the next
new record on the subform.
Any help much appreciated.
Thanks