Is there a way, using an update trigger, to effect changes to a different table when a value in another table is changed?

For example:

Parent Table/Field: CLIENT.acctbal

Child Table/Field: TRANSACT.amount


Is there a way to edit an existing record in the TRANSACT table, and reflect the change in the CLIENT table? I don't see how to know what the original value was before the Update Trigger was called.

Thanks for any insight,
Chris