“But our rewards will be eternal not temporal. “
Yes and no. The primary rewards are eternal.
But don’t forget Matt 19 where the disciples told Jesus they had given up everything to follow Him. He said they would receive all that and much more, in this life and the next. This was a consistent theme in the OT as well.
As for the widow, she gave out of her poverty, but we are promised that God would open the windows of heaven and pour out blessings when we give tithes and offerings.
God does not like to be in debt to anyone. The OT says, “Who has ever given to God and God has owed him the debt?”
Matt 19 is pretty clear. Jesus answered His' disciples by telling them to forsake all their worldly things and they'd be rewarded with spiritual things. Jesus certainly didn't tell them to go and build mega-churches and keep the doors locked 5 days a week.
"but we are promised that God would open the windows of heaven and pour out blessings when we give tithes and offerings."
It seems as though you and CyberAnt (who quoted Deut 28) are hung up on the "conditional & temporal" promises God made solely to the nation of Israel. We are not Jews, we don't live in the covenant land, we're not under the condemnation of the law given to Moses, and we certainly have a perfect Priesthood compared to the Levites. (Heb 10:1-10)
The Israelites were not able to receive the promises/blessings because of their disobedience. What makes you think we would fare any better under the conditions of the old covenant? If you are trying to receive the blessings God promised Israel (by quoting Malachi) then you must also partake of the curses, too.