alter FUNCTION RET_xxx (@val varchar(20),@max)
returns varchar(20) -- <======= !!!! use CHAR(5)
AS
 RETURN RIGHT('00000'+RTRIM(@val),5)
END

alter FUNCTION RET_xxx (@val varchar(20),@maxlen TINYINT)
returns VARCHAR(<maxLenght>)
AS
 RETURN RIGHT(REPLICATE('0',@maxlen)+RTRIM(@val),@maxlen)
END