If you have this relationship:
LayoutTable::ThisYear = Reservations::Year
then you can either:
Define a calculation field in LayoutTable as Sum ( Reservations::InvoiceTotal )
or define a summary field to compute the total of InvoiceTable in Reservations and then place this summary field on your LayoutTable's Layout
Both will show the same total. The relationship limits the total to just Reservations made in the year specified in ThisYear. ThisYear can be a number field (possibly global) where you specify the year or it can be an unstored calculation field defined as:
Year ( Get ( CurrentDate ) )
if you want to "hardwire" it to always match but the current date's year.
This may require creating a new Tutorial: What are Table Occurrences? of Reservations in order to set up a relationship that does not interfere with existing relationships linking LayoutTable to Reservations.
The ExecuteSQL() function (not the script step) could also compute this total without modifying your relationship graph.
Thanks PhilModJunk for responding. I feel that your suggestion has further complicated my thinking but will work on it!