*shall I give my land to other developer directly without asking old developer?
The way to go about it to contact your existing developer and clearly discuss the situation.
*is it necessary to give back money which is given by lease?
If the land has not been used/ misused and if both of you agree, you may want to deduct a reasonable amount and hand over the rest to the existing developer.
*what is the power of this unregistered lease agreement?
unregistered lease deed is invalid in law and not an admissible evidence
*now old developer is not ready to start work
Discuss face to face, settle the dues and hand over the property to the new developer.
Being legally correct is one thing and being a sincere person is another, take time and decide wisely.