This would be the scenario when an Assignment Agreement will come in:
"A" is an investor. Initially "A" enters into a Sale Agreement with the builder during pre-launch or when the property is under-construction. "A" may or may not have taken a home loan for this transaction. "A" keeps making payments to the builder. Then when the building is ready to move, the builder asks "A" to get the Sale Deed registered. But "A" has already found a buyer "B" for his flat. However "A" is still not the owner of the flat as the Sale Deed is still not registered in his/her name. This is when a tri-party Assignment Agreement will be made between A, B and the Builder. The Assignment Agreement will essentially ensure that the Property gets registered directly in the name of "B" where "A" should be a consenting party. It will also have some terms and conditions specific to this transaction.
Hope this helps.
Kris