Sum for group in memory

Hello everybody:

I need some help, I have a tabular datawindow and it´s a data entry, it has a money field and I have to group for 3 fields and sum the money field,

the problem is the data is not in database for doing a sum group by, by the way if the sum is less than 0 for any grouping I can´t save the data. Can I sum in memory  or something like that.

I will appreciate any help

Regards

SandraE