God is not mean (Hebrews 6:10).  He does not abuse His servants nor ill-treat His friends.  He rewards those who give service, time, labour, money, possessions and worship in His Name (Matthew 19:29).  But God never defines a particular reward for individual acts of sacrificial giving.  Perhaps that is because we are not employed by Him on a contract; so we are never given the luxury of predicting what we get for what we give.  Neither are we to compare our reward with anybody else's.  Nor are we allowed to know when or how God's reward will arrive - in time or eternity.  

However, Jesus wanted His disciples to be clear about the quality and quantity of His reward.  It is always good, and always exceeds what is given.  God is not limited by human ideas of what is best: as Ephesians 3:20 says, He is: ‘... able to do immeasurably more than all we ask or imagine, according to his power that is at work within us, ...’ As apostles, they should teach the church not to hold back from giving through fear of being reduced to nothing.  The more they would give, the more God gives.  Indeed, all we have is God's: as we pass it on, He is glorified, and we receive further provision.  The more we pass on, the more He gives us to give to others (2 Corinthians 9:6-11).

Ultimately, our reward is not measured in possessions but in honoured relationship with Jesus (Colossians 3:24; Revelation 22:12) and the responsibilities Jesus will give to us (Matthew 25:14-30).

We need to be encouraged to give.  The promise of reward is such an encouragement (Hebrews 11:26).  However, unfortunately, this principle has been misused by some leaders who demand money with promises that God will give more money in return to make the giver more prosperous.  But there is no evidence of the Lord Jesus or the apostles ever being rich in money, although they were rich in faith, hope and love.  It should be obvious that if we give because of what we can get, then greed and not grace is operating in our hearts.  But do not let the abuse of Jesus' words stop us from being obedient.  If we can respond to genuine need in the Name of Jesus, we should do so gladly without thought for our own welfare; and simply trust the Lord to deal graciously with us in this life and the next.