How can I force a delimited data source to not skip blank values in an Amount field?

There is the Fixed Debit Credits parser script in GolfStream that might give you a clue:

Namespace OneStream.BusinessRule.Parser.XFR_ParseFixedDebitsCredits
	Public Class MainClass
		'------------------------------------------------------------------------------------------------------------
		'Reference Code: 	XFR_ParseFixedDebitsCredits 
		'
		'Description:		Parse a fixed numeric column based on a mid-point position in order to determine debits and credits.
		'					This script is required for file formats that have values in both the Debit and Credit column.  It performs
		'					extra tests to if the debit column is zero, if so it moves on to the take the credit column as the value.
		'
		'Usage:				Parser business rule intended to be used on numeric field containing a values for debits and credits.
		'
		'Created By:		Tom Shea
		'Date Created:		1-13-2014
		'------------------------------------------------------------------------------------------------------------				
		Public Function Main(ByVal si As SessionInfo, ByVal globals As BRGlobals, ByVal api As ParserDimension, ByVal args As ParserArgs) As Object
			Try
                'Make sure the line is longer than the amount field position + length of field
				If (args.Line.Length >= (api.DimensionStartPosition + api.DimensionLength - 1)) Then				
					'Declare the Debit/Credit midpoint position
					Dim midPoint As Integer = 12
					Dim fieldValue As String = Mid(args.Line, api.DimensionStartPosition, api.DimensionLength)
									
					'Parse the Debits & Credits in order to return the NON-ZERO value
					If fieldValue.Length > midPoint Then
	                    Dim strDebit As String = fieldValue.Substring(0, midPoint).Trim
	                    Dim strCredit As String = fieldValue.Substring(midPoint, fieldValue.Length - midPoint).Trim
						
						If IsNumeric(strDebit) And IsNumeric(strCredit) Then
							If CType(strDebit, Double) = 0 And CType(strCredit, Double) = 0 Then
								'Both a values are non-numeric, just return the incoming value so we can log it
								api.ValueIsZeroSuppressed = True
								Return "0"						
							Else
								If CType(strDebit, Double) <> 0 Then
									'Debit is not zero
									api.ValueIsnumeric = True
									Return strDebit
								Else
									'Debit was zero, so return the credit
									api.ValueIsnumeric = True
									Return strCredit & "-"
								End If
							End If
						Else If IsNumeric(strDebit) Then
							'Debit is a valid number
							api.ValueIsnumeric = True
							Return strDebit
						Else If IsNumeric(strCredit) Then
							'Credit is a valid number
							api.ValueIsnumeric = True
							Return strCredit & "-"
						Else
							'Both a values are non-numeric, just return the incoming value so we can log it
							api.ValueIsnumeric = False
							Return args.Value
						End If                    
					Else
						'Value is less than mid point, just return the incoming value so we can log it
						api.ValueIsnumeric = False
						Return args.Value		
	                End If
				Else
					'Line is blank or not long enough
					Return args.Value
				End If
				
				Return Nothing
			Catch ex As Exception
				Throw ErrorHandler.LogWrite(si, New XFException(si, ex))
			End Try
		End Function
	End Class
End Namespace