Updating Sforce currency field problem

Hello,

I'm struggling to update a currency field in Salesforce. Here is my code:

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Dim o As SalesForceAPI.Opportunity = New SalesForceAPI.Opportunity
o = opp
o.Actual_Revenue__c = CDbl(revenueRow.SUM_RECEIVED)

Dim sr As SalesForceAPI.SaveResult() = sforce.update(New SalesForceAPI.sObject() {o})
For d As Integer = 0 To sr.GetUpperBound(0)
    If sr(d).success Then
        Console.Write("Success")
    Else
        Console.Write(sr(d).errors(0).message.ToString & vbCrLf)
    End If
Next d
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I tried hard-coding a number in place of revenueRow.SUM_RECEIVED in the above code for debugging purposes but that did not work either.

If I change the sforce field type to text and remove "CDbl" from the above code the update works perfectly. Any advice how I can insert a numeric value into the currency field, Actual_Revenue__c?

Thank you in advance for your help!
Mandy