I suppose that you just replaced the date(...) function by the name of your variable. It does not work because, inside a "select"  statement, the context is not the current record of the table you are working in, but the records of the table you are searching in. So you need to save the value of the field in a variable while in the context of the table you are working in before executing the "select".

let Zdi := 'Data Incassi'

sum(...... 'Record Date' = Zdi .....)