Hey, Thanks for using the asset, i hope it is being useful for you!
About the donations, as long as the project is not primarily meant to generate income ( Paid, micro transactions and etc) i have no problem with you being able to receive donations, you can use the asset for sure.
But if you plan to have the assets in the final version (paid one) i would recommend you to buy the paid version of the asset to get the licensing and the other paid exclusive content, but that's just the case if you're using my assets in a paid game, there's no problem about using it as a temporarily placeholder or it being present in demo's or any other free to play media.
If you have any other questions don't forget to send me a message, and good luck with your project!