TotalAmount is a Numeric
I try
let Update = SQL("Insert into [dbo].[Facturen] ([FactuurNr], [Date], [TotalAmount]) Values ('" + CStr(iFactnr) + "' , '" + CStr({?dsFactDate}) + "' , '" + 120.25 + "')")

and

let Update = SQL("Insert into [dbo].[Facturen] ([FactuurNr], [Date], [TotalAmount]) Values ('" + CStr(iFactnr) + "' , '" + CStr({?dsFactDate}) + "' , '" + {@Total} + "')")