One way to do this is via a Domain Aggregate Function. This can be done
in the query that the form is based on, or in the Control Source of the
textbox on the form. In the latter case, something like this...
=DSum("[TheAmount]","YourTable","[TheSortingField]<" & [TheSortingField])
If you have a very large number of records, the loading of the form may
be a little clunky.