Many charities and places of worship do valuable work that's worth supporting. Since it makes sense that God would be pleased with financial support for organizations that do good work, some people think that God thanks those who give money by giving them miracles.
The idea of getting something in return for giving is problematic because it contaminates the purity of your motivation for giving. When you give money expecting to get a miracle in return, your focus shifts from blessing others to blessing yourself, moving you away from love and toward selfishness. Giving to get is also spiritually dangerous because it amounts to trying to bribe God rather than trusting in his plans for you. Most religions urge people to give without expecting any material rewards like money, but that doesn't mean that your giving will go unrewarded.
Most of the world's religions say that God rewards those who do good deeds with a pure motivation (to express their love for God and other people) by giving them spiritual blessings such as greater compassion and wisdom. You may consider those spiritual blessings to be miracles when you allow them to change the kind of person you are, since personal transformation for the better can be a miraculous gift.
So when you donate money to a charity or place of worship, try to give in the way God himself gives: freely, motivated by love. Then you can experience the miracle of becoming more like God in the process.