Firslty, as you have mentioned that others have been agreed upon giving their share to you for a consideration, and that is also in two instalment.
Secondly, for that they will have to execute a release/relinquishment deed in your favour.
Thirdly, over which you will have to mention that you would be paying first instalment on such and such date and thereafter on another date the second instalment.
Fourthly, once you have done so then you would be safe as well as you would be having a proof for the same.