you should walk very carefully. if he has given the cheque directly to the hotel, reception house etc. then, it is not refundable and no sweats for you. but if he had given a cheque to you in your name, then they may level the allegation of receipt of dowry and a dowry case is round the corner. so, make yourself clear on that point to proceed further. Now alternative avenue, I show to you. if she insists for divorce, you may also can file a divorce case on the ground of adultery, mentioning the paramour's name, which ultimately put them in defence to protect their family honour and dignity intact and may accept to your terms for giving divorce. on the other hand, if you have sufficient proof of adultery, you can file a criminal case against the 'wifejack' (adulterer) and that will also push your wife and her father in a tricky corner and when the chickens come home roosting, your in-law will chicken out and give you a leeway to extricate from this day/night-mare marriage. So tread cautiously.