I think a good way to go about is a 'request/approve' form handled through the Nexus for example. Say I was to ask Author1 for permission to port his Mod1 from Skyrim to SkyrimSE (for example), I fill a 'request' form on the Nexus, which has a link to Mod1 and states that I am interested in porting it from one game (origin) to a different game (target). I fill the form, which has that info, as well as details on the permissions (it could just default to 'whatever the mod currently has as permissions' unless stated otherwise), and it could include a message as well to include other details. Once I am done, that form is sent to Author1, who gets a Nexus notification (on the website, or through email as well, kind of like how PMs work at the moment). Author1 then can review the form on the Nexus, and if he agrees he 'approves' that request. If he doesn't agree he can chose to 'ignore' it or 'negate' it. He can also return the request further detailing conditions, such as 'you can port it but you can't do further modifications', or 'you can port it and modify/improve it if you want', stuff like that. Since all of this is handled through Nexus servers, they have proof of the exchange and can log it into their database if they want, depending on how they code the forms. I think that would be far better than a screenshot which can easily be photoshopped by anyone.