Is it permissible to return a domestic worker to the agency out of fear of cutting off her livelihood or oppressing her, especially after a suspicious act was found from her (putting urine in tea), given my knowledge that the contract allows me to return her within 3 months?

There is no objection to requesting a replacement for the maid or returning her if her situation is as described, and the contract allows for this.

Her return or replacement does not constitute injustice to her or cutting off her sustenance, for Allah alone is the Provider, and no one can prevent sustenance that Allah has decreed for His servant. Allah, the Almighty, says: "Whatever mercy Allah opens for people, no one can withhold it. And whatever He withholds, no one can release it thereafter. And He is the Exalted in Might, the Wise."

A servant will not die until his sustenance reaches him in full. And if the entire nation were to gather to benefit you or harm you, they would not benefit you or harm you except with what Allah has decreed for you or against you.