This is a case where you first get the two of them in a room, and let them hash it out.
In an Agile workflow, the product owner defines the requirements. The team lead has a voice while the story is being groomed (as should all team members), but once the story is in the sprint the requirements should not change.
If the two of them cannot come to an agreement\understanding, bring it up with your manager, as this may be an issue you cannot resolve yourself.
At the moment though, it appears your team lead needs to be reminded that the product owner defines the requirements and it is up to the team as a whole to make it happen (implement).
Note: If there is a technical reason that a requirement may need to change, that should be discussed with the product owner before coding.