I remember reading of a custom in the Roman Republic where a pact could be sealed between individuals with a broken pot, or wax tablet, whereby the pieces could be later be reassembled to show this bond. The name of this tradition, or some other signifying factor would be appreciated.
Not Roman, but Greek. The term you are seeking is Symbolum. The best description I find is from the book Everything is Sacred: A Complete Introduction to the Sacrament of Baptism By Thomas J. Scirghi. (emphasis mine):
The word symbol derives from the Greek word symballein, literally meaning 'to throw together.' In ancient times, a symbolum was used to establish a contract. When two parties entered into a contractual agreement, they would cut an object into two parts and each of the parties would retain one of the parts. For example, the parties might break a piece of pottery into two pieces, and each of the parties would hold one piece. When they or their representatives met again, they would present their individual pieces, and the broken, jagged edges would interlace, indicating they were both part of the same vessel. The reunited object was the symbolum.
So literally the broken shards become symbolic of the contract created.