Validating controls

Validating controls

I have a control on the [parent form purchases] which sums the total value of goods received note from the tables (txtgrnValue). This control is used to net off with the purchases subform (txtgrandtotal), for simplicit purpose see below:

Txtgrnvalue – txtgrandtotal = 0 (txtbalance)

If the net of the above two controls is zero (txtbalance), then it is safe for the users to proceed to the next sub form which deals with double entries.

Now assuming the net of the two is not zero (txtbalance), I want to stop the users from proceeding to next sub form until the above control is balanced off to zero.
How do I do it????
Which event to use???????


Regards
Chris