1. It is perfectly normal for the buyer to pay 90% balance amount at the time of registration. The buyer may not agree to pay the maximum amount before the deed is registered since he also will want to be protected from any of your act which will amount to denial of registration of the sale deed,
2. You can take photocopy of the DD and get it verified from the Branch of the issuing Bank to feel 100% sure of its authenticity,
3. You can have the said clause featured in your sale deed though the said clause stays there by default. You should mention the DD number in your said sale deed. You need not be so sensitive to have a copy of the said sale deed since you can get the certified copy there of from the Registrar's office. However, you can have a photocopy of the same,
4. You shall take the draft before registering the sale deed. If the said DD is physically damagwed, then do not sign the sale deed before the Registrar. After receiving the DD and completing the registration process, keep the DD properly to deposit to your account.