In 5:12 the Torah begins telling the laws of Sotah. Rashi comments on the proximity of the Sotah section to the previous section discussing giving the priestly gifts. The previous section concluded by saying one who gives to the Kohen hsi portion, will have what to give, meanign he will be blessed with wealth and will have more to give in the future.
Rashi comments on the proximity of these two sections and says that the lesson to be learned is if you give your priestly gifts, you will be blessed with wealth, but if you try to aabstain from giving your gifts and keep it for yourself, you will end up needing to come to the Koen and bring him your Sotah (your wife for judgement of Sotah and the korban of sotah).
This is a lesson we commonly find in Judaic tradition. We were always told as children and as students that if one does not give the charity he is supposed to give and tries to get away with giving less, something will happen that will make you lose that money anyway. If you "owe" $500 to charity but only give $100, your fridge will break down and you will spend a lot of money in repairs, for example.
If we think that the money is ours and we earned it and can do with it as we please, then we also feel why should I give it away. I will be charitable and give a little, but why give so much? I need it to! If that is how you feel, then Hashem will find ways to take it away and show you that it is not yours, rather He gives us money but wants certain things in return.
If you do not give what you are supposed to, God will take it from your other pocket.