A Center Controller reported an aircraft climbed from their assigned altitude without the Center Controller's knowledge. A procedure in a new LOA with the adjacent Center allows the adjacent Center to issue a CPDLC clearance to an aircraft not yet in their airspace.

Narrative

Aircraft X was level at FL370 with no CPDLC connection to KUSA nearing the Canadian border. An automated handoff was initiated to Winnipeg Center Sector XX and they accepted radar several moments later. Upon attempting to ship Aircraft X to [Winnipeg] Sector XX via voice; they did not respond on the first attempt. On the second attempt; the pilot responded that they would be contacting Winnipeg Center and climbing to FL390. I was taken aback because this control instruction was not issued by me and I told them to maintain FL370. They responded that Winnipeg Center had already approved a climb to FL390 via their CPDLC system (still in [United States] Sector XX airspace but this is authorized by the new LOA). Since there was no traffic conflict; I replied roger and shipped them. I felt uneasy about the situation because while there was no conflict and this scenario will likely not happen very often; I feel like the new LOA with Winnipeg in which they can change altitude and 45 degree turns could; in very specific scenarios like this one; lead to a potential loss of separation. If I was deliberately holding onto Aircraft X to ensure they were clear of traffic in my airspace but they were already logged on to Canadian CPDLC and receiving control instructions from them; it has the potential for disaster. Recommendation: I do not have a solution to this but it's obvious that an aircraft should not be receiving conflicting control instructions from 2 different facilities at the same time. ZMP does not know when an aircraft is logged into Canadian CPDLC nor does Winnipeg know when they're logged into ours. In this case; Aircraft X was never logged into KUSA but WAS logged into the Canadian system. Perhaps an amendment to the LOA should include a clause that Control for Altitude Changes and turns in the adjacent sectors airspace can only be accomplished when requested by voice by the pilot and NOT CPDLC until the aircraft is in the new controller's airspace. This will ensure that the 'shipping' controller has full resolved any conflicts for that aircraft and has told them to contact the next sector