Error converting varchar to decimal

create PROCEDURE [dbo].[dagn_InsertTransaction_v1]
@Ref1 AS VARCHAR(50)
,@Ref2 AS VARCHAR(50)
,@Ref3 AS VARCHAR(50)
,@Ref4 AS VARCHAR(50)
,@Ref5 AS VARCHAR(50)
,@BillerID AS VARCHAR(50)
,@TxnAmount AS DECIMAL(28,2)
,@txnCurrencyCode AS VARCHAR(50)
,@txnCountryCode AS VARCHAR(50)
,@agentcode AS VARCHAR(20)
,@channelcode AS VARCHAR(20)
,@agentiduserid AS VARCHAR (100)
,@version AS VARCHAR(20)
,@latitude AS VARCHAR(20)
,@longitude AS VARCHAR(20)
,@productdesc AS VARCHAR(20)
,@servicefee AS DECIMAL(28,2)
,@oneStopfee AS DECIMAL(28,2)
,@approvalCode AS NVARCHAR(50)
,@AgentBranchCode AS VARCHAR(50)
,@BatchID AS INT
,@IP AS VARCHAR(20)
,@appType AS varchar(2)
,@msgid As varchar(255)
AS


insert into TBL_TRANSACTION (
REFID1 ,
REFID2 ,
REFID3 ,
REFID4 ,
REFID5 ,
BILLERID,
TRANSACTIONAMOUNT,
...