Has anyone done an agreement with their illustrator OR as an illustrator with their Author.

Just a little background:

My children's book, 'Tails of Oreo" is being printed and my illustrator was chosen because he and I worked together in print media.
He was a graphic artist, but had background as an illustrator, and was anxious to work with me on the book.

I paid him for each illustration as he completed them, until he was done. So all his illustrative work was paid for upfront.

Now he once an agreement as we approach another book, which we hope will be a start of the series, and he is asking for royalties.

Can anyone share a similar situation or know of how this works OR NOT!!

If you can help, my phone is 408/849-7558 for text.