Must declare the scalar variable "@PreviousAmount"

Here's what i did

Declare @PreviousAmount money   
set @PreviousAmount = 0

select @PreviousAmount  = @PreviousAmount  + Inp.AmountPaid
    From Invoice as Inv , InvoicePayment as Inp
    where Inv.InvoiceId = Inp.InvoiceId 

and I got a error saying "Must declare the scalar variable "@PreviousAmount"."

Updated - This is actual code which gives error

CREATE PROCEDURE [dbo].[spInvoiceTsk]
        @AmountPaid money 
AS
BEGIN
    SET NOCOUNT ON;
    Declare @PreviousAmount money 
    set @PreviousAmount = 0

select case 
        when (@AmountPaid = Inv.InvoiceAmount )
         Then @AmountPaid 

        else (
        select @PreviousAmount  = @PreviousAmount  + Inp.AmountPaid
        From dbo.Invoice as Inv , dbo.InvoicePayment as Inp
        where Inv.InvoiceId = Inp.InvoiceId
        )
        from dbo.Invoice as Inv

        --select *
        --from Invoice as a
        --Inner Join InvoicePayment as IP ON a.InvoiceId = IP.InvoiceId

    update I
    set I.AmountPaid = @PreviousAmount
    from Invoice as I , InvoicePayment as IP
    where I.AmountPaid = IP.AmountPaid

END
GO