In "Doctrinal Discussion" Jason Lindahl writes:
Jesus said, “Give, and it shall be given unto you; good measure, pressed down, and shaken together, and running over, shall men give into your bosom. For with the same measure that ye mete withal it shall be measured to you again” (Luke 6:38). Human nature being what it is, if people are led to believe that certain circumstances excuse them from tithing, they will tend always to find themselves in those circumstances! Instead of encouraging people to withhold more because they are poor, we should encourage them to step out in faith and trust a never-failing God to bless them for keeping His commands. “Bring ye all the tithes into the storehouse, that there may be meat in mine house, and prove me now herewith, saith the LORD of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall not be room enough to receive it” (Mal. 3:10).
Source: Building Christian Relationships: Adult Teacher's Insights, page 38.