I think Abhi is not the real Abhi, Manno Bhabhi's son. He is in fact Shekhar Sharma and Manno Bhabhi's husband brought him in to impersonate Abhi. Their plan is to garner sympathy and someway try to grab Samar's property as well as Brijesh-Nahar's property. The papers Saloni saw are probably a deed of property transfer in Shekhar Sharma's name. Manno Bhabhi's husband may have given them to him as an initial payment for his services.
As for the real Abhi, there are many possibilities.
1. He may be dead long ago. Manno Bhabhi is unaware of it and hence her husband is taking advantage of it by putting someone else in his place.
2. He may be alive and still in the US, completely unaware of what's going on back home.
3. He may be in India but may have refused to participate in his father's schemes and also threatened to tell the truth to Manno Bhabhi. So may be he is being kept a prisoner by his father and Shekhar Sharma to stop him from spilling the beans.
Personally, I would prefer the third option. After Saloni exposes his father and the fake Abhi, I would love if the real Abhi comes forward and Manno Bhabhi finally gets a son she need not be ashamed of 😊
Your thoughts are welcome😛