First of all, you NEVER give a project vendor full payment. Always in stages on agreed upon checkpoints/milestones.
Second: If the POS company will not honor the agreement (won't return calls etc), it is time to bring in the lawyers to do their dirty work to attempt to recover some of the investment, or make them finish up.
Third: I know this sounds strange, but if there is proprietary hardware/software involved, the contract signed may forbid you from doing modifications to the hardware and or software. Even though they absconded with the $$, they could possibly come back and sue YOU for violating the EULA contract. Bring the lawyers in.
Fourth: Good luck! This is definitely a no win situation
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