Spirit of the Law?

Is it proper to circumvent Halachic requirements?  We sell chametz to non-Jews in order to circumvent the requirement to get rid of it; we write a prozbul in order to circumvent the cancellation of loans by shemittah.  Doesn't the Gemara bemoan the practice of bringing in produce in a way that technically exempts it from the requirement of maaser?  Are there criteria by which we can determine when it is appropriate to technically circumvent Halachic requirements and when it is not?

The Limits of Debt

Is a person entitled to give money to charity if he owes money to someone else?  If not, may a person give money to charity if he does not pay full tuition for his children's education?

The Source of Goodness

Religious people assume that it is good to fulfill the Will of God.  Is that because God wants us to do that which is good, or because the fact that God wants us to do a certain thing makes that thing good?  Is there an independent source of goodness to which the Will of God conforms, or does God's Will define goodness itself?