Firstly create a basic measure [Amount] to sum the dollar amount column. I then used that within RANKX() to create the following:
[Rank] = RANKX(
ALLSELECTED(Table1),Table1[Claimant Number]=max(Table1[Claimant Number])
The key is the table that the [Amount] measure is iterated over - ALLSELECTED() just brings the stuff in the current filter context into play and the expression within the FILTER() restricts the table to the current claim number.
After that it was a simple task to return the [Amount] based on whether or not the [Rank] was 1:
[Opening Balance] = if([Rank]=1,[Amount],BLANK())