How did justice Marshall justify ruling that Dartmouth's charter was a contract?

The Contract Clause in the Constitution was used to decide the case of Dartmouth v. Woodward. A contract was stated to be any transfer of property between peoples and does not include government politics.

Though Dartmouth College received their charter and land from the government, the contract is between the individuals of the government and those taking ownership of the college. The Marshall Court decided that under the Contract Clause, Dartmouth College had been given the land and were unable to take the charter back out of private hands.