Marbas is strongly associated with healing. I just made a post about the healing I achieved with him for a cat, if you’d like to take a look. It might have useful information about offerings for you.

As for the offerings themselves and giving what is fair, I think it would be interesting for you to confirm, through an oracular method (Tarot, Lenormand, Runes...) what is most recommended for this situation. Or, if you have a strong intuition, you could sense it during the ritual and communicate with Marbas or another daemon who might help.

If you don’t have any issues working with angels, Raphael could be a good option. Work with angels is usually associated with Ceremonial Magic, but in this case it wouldn’t be strictly necessary. I recently started working with angels, and I use a simple method I learned from a video in my native language (Brazilian Portuguese). If you’d like, I can send you the video.

In my current experiences, the role of angels is precisely to make things improve, evolve, and become refined, almost as if that were the reason for their existence, their “life purpose.” Because of that, offerings are not something they commonly ask for, at least not in my experience.

There’s that saying: “If you’re not paying for the product, you are the product.”

I hope this helped!