这个很简单，您可以设置汇总模式：

http://www.foxtable.com/help/topics/00144.htm

应收金额小计 - 收款金额小计 = 未收金额小计 如何设置

用自定义按钮汇总吧。

Dim t As Table = Tables("应收款")
Dim g As SubtotalGroup
t.SubtotalGroups.Clear()
t.GroupAboveData = False
t.TreeVisible = False
t.SpillNode = True

g = New Subtotalgroup
g.Aggregate = AggregateEnum.Sum
g.GroupOn = "客户"
g.TotalOn = "应收金额,收款金额"
g.Caption = "{0} 小计"
t.SubtotalGroups.Add(g)

g = New Subtotalgroup
g.Aggregate = AggregateEnum.Sum
g.GroupOn = "月份_年"
g.TotalOn = "应收金额,收款金额"
g.Caption = "{0} 小计"
t.SubtotalGroups.Add(g)

t.Subtotal()
Dim r As Row
For i As Integer = 0 To Tables("应收款").Rows.Count(True) - 1
    r = Tables("应收款").Rows(i,True)
    If r.IsGroup
        r("未收金额") = r("应收金额") - r("收款金额")
    End If
Next

谢谢！