You can rectify mistake in DD number by execution of deed of rectification 

it should be stamped and registered 

As such there is no court at kashmere gate. There is registrar office. Sale deed is required to be rectified so as to avoid future complications. 

For execution of deed of rectification presence of both parties is necessary 

first party can execute POA in favour of family member to execute deed of rectification 

If first party is not available for Rectification Deed to be executed by him, execute a Declaration/Declaratory Deed declaring clearly regarding the DD details and register it in the same Delhi Kashmere Gate Registrar's Office.

There's no need of first party for registering the Declaratory/Declaration Deed.

If it's possible for both the husband and wife to execute a registered Power of Attorney to one of their parents/siblings or to you, then the person so appointed through POA can execute the Rectification Deed in your favour in the registration office.

Yes, correspondence. 

Since execution of Rectification Deed is out of question, Declaratory/Declaration Deed is suggested, where you have to narrate the circumstances that led you to execute the Declaratory/Declaration Deed. 

Anyway that DD amount has been acknowledged by the sellers and have acknowledged it.

When the sellers return to India, then get the Rectification Deed done and till such time, the Declaratory/Declaration Deed will be in its place.

This is just an error due to bank however the bank has rectified the mistake and issued a fresh DD and the sale deed also was registered in the presence of the seller who acknowledged the receipt of the sale consideration amount. However in order to avoid any claim for consideration amount in the future, you can obtain a receipt duly witnessed and notarised from the seller stating the details of both the DDs and keep them along with the original sale deed on your name safely.

This will prevent any future legal issues that may arise in future with regard to the payment of sale consideration amount.

There is no necessity for the first party in order to get a rectification deed executed, however you find him and get a notarised acknowledgment for the receipt of the sale consideration amount paid through another DD for this purpose.

The second option other than the rectification deed, is the one what I suggested, i.e., to obtain a notarised receipt for the payment received towards the sale consideration amount by a different DD instead of the original DD as mentioned in the original sale deed.

For filing a declaration suit to declare the sale deed as valid by paying the sale consideration amount by a different DD instead of the one mentioned in the original sale deed is cumbersome process.

For that, you have to first issue a legal notice to the seller asking him to execute a registered rectification rectifying the erroneous DD number, he should refuse or should not take the communication and remain silent about it after which yo may have to approach the concerned sub registrar with a deed for  rectifying the error.

The sub registrar has to reject the deed after which you have to approach court with all the documentary evidences with a suit for declaration to declare the correct DD number and the same to be rectified by a registered rectification deed.

Therefore you may decide about handling the same in a easy and legally valid mode as suggested.

For rectification deed both parties are needed otherwise atleast their legal heirs needed

Ask them to execute POA in favour of family member or friend 

get POA attested before Indian consulate 

on basis of POA deed of rectification can be executed 

Deed of declaration is unilaterally executed by you and registered 

whereas in deed of rectification both parties execute the deed to rectify mistake in sale deed 

- You can rectify this mistake after executing a registered rectification deed , however the presence of both the parties are necessary for registration 

- Further, if they are out of India, then they can give POA to any relative to perform the same on their behalf. 

- However, even there is no requirement for the rectification in the sale deed if already it registered , and you can take from them a confirmation of the payment , like a declaration or receipt of the amount , and this document must be signed by both the parties in the presence of two witnesses. 

- They can send that declaration/receipt dully notarized by the notary of that country..

Dear Client,

A mistake in a registered deed can be rectified through the execution of a registered rectification deed. Both parties must be present during the registration process. However, if either or both parties are currently outside India, they have the option to grant Power of Attorney (POA) to a trusted relative, who can act on their behalf for the registration process.

It is important to note that if the sale deed has already been registered, there might be no need for rectification. Instead, a confirmation of the payment can be obtained from both parties in the form of a declaration or receipt, duly signed in the presence of two witnesses. To ensure its validity, this document can be notarized by a notary in the country where it was executed. This process allows for rectification even when both parties are not physically present in India.