I learnt that giving of offering or contributions are done according to what one purposes in his heart…

Yes it was preach by brother Eli very clear that we must obey the doctrines of the Lord Jesus Christ and tithing is not part of the doctrine. We should give according to the will of the heart, not grudgingly or in necessity because God loves a cheerful giver.

But many pastors from other churches is using the tithing to earn money from their members.

God bless you brother @kingly0