How to solve this Error ' converting data type nvarchar to float.' while inserting a record in sql using stored Procedure

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SQL

C#5.0

I am getting error 'Error converting data type nvarchar to float.' while inserting a record in the table using stored procedure.

My stored Procedure is

USE [ezNew_db]
GO
/****** Object:  StoredProcedure [vipl-user].[SP_Copy_Bill_InsertData]    Script Date: 01/06/2015 09:31:32 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER proc [vipl-user].[SP_Copy_Bill_InsertData](@tablename nvarchar(50)  ,@INV_NO nvarchar(50),@AC_NO nvarchar(50),@AC_NAME nvarchar(255),@INV_DATE nvarchar(50),@AD1 nvarchar(255),@AD2 nvarchar(255),@AD3 nvarchar(255),@AD4 nvarchar(255),@AD5 nvarchar(255),@AD6 nvarchar(255),@TERMS nvarchar(30),@EXCHANGE_RATE float,@CREATED_BY nvarchar(10),@DATE_CREATED nvarchar(50),@ZERO_RATED_INV bit,@TAX_INCLUDE bit,@Deleted bit,@SEAL_NO nvarchar(50),@JOB_NO nvarchar(255),@Inv_Amt nvarchar(50),@GST nvarchar(50),@Inv_Net nvarchar(50),@Status nvarchar(50)) as
begin
Declare @sql nvarchar(max)
set @sql='insert into '+@tablename+'([INV_NO],[AC_NO],[AC_NAME],[INV_DATE],[AD1],[AD2],[AD3],[AD4],[AD5],[AD6],[TERMS],[EXCHANGE_RATE],[CREATED_BY],[DATE_CREATED],[ZERO_RATED_INV],[TAX_INCLUDE],[Deleted],[SEAL_NO],JOB_NO,Inv_Amt,GST,Inv_Net,Status) values ('''+@INV_NO+''','''+@AC_NO+''','''+@AC_NAME+''','+CAST(@INV_DATE as varchar(50))+','''+@AD1+''','''+@AD2+''','''+@AD3+''','''+@AD4+''','''+@AD5+''','''+@AD6+''','''+@TERMS+''','''+CAST(@EXCHANGE_RATE as float)+''','''+@CREATED_BY+''','+CAST(@DATE_CREATED as varchar(50))+','+CAST(@ZERO_RATED_INV as CHAR(1))+','+CAST(@TAX_INCLUDE as CHAR(1))+','+CAST(@Deleted as CHAR(1))+','''+@SEAL_NO+''','''+@JOB_NO+''','+CAST(@Inv_Amt as decimal)+','+CAST(@GST as decimal)+','+CAST(@Inv_Net as decimal)+','''+@Status+''')'
exec(@sql)
end

I am doing my project in 3 tier I have taken Exchangerate field in BEL as float/decimal.

Would you Please resolve this error.