Legally in India there is no provision for a contract marraige.We have not seen them sign any papers it is mainly verbal agreement. Shyam only knows that Arnav maaried Khushi holding Payal's marriage as a collateral which at this point is not relevant considering what the family thinks of Shyam. Nani and Anjali knows that Arnav had to get married quickly due to Shyam's faisco. So probably Anjali or Nani will suggest a small religious ceremony to please Dadi. Highly doubt a large scale wedding taking into account Anjali is due to have a baby soon and Khushi /Arnav are already living together as married couple.