Any government is allowed to choose to do whatever it wants however it wishes; in a republic: given what is legally determined by the three branches. Obviously. They can contract with whomever they want, make any deal with whomever they want.

This also means that they should adhere to a deal once it is signed. That's part of the law too. They shouldn't suddenly turn around and try to alter the deal, then retaliate against their deal partner when they say "that wasn't the deal". You can't just go and answer: "Pray we don't alter it further".

The government of a nation sets the example for others, and should be scrupulous in their dealings.

Just because you sign a deal that didn't mean you can't change it or terminate it in the future. As long as both parties agree any contract can be modified or terminated. If they don't usually the contract contains information on how it works.