Microsoft Volume I

Ben Gilbert & David Rosenthal says

IBM paid Microsoft $430,000 fixed, bundled together from $775,000 for testing consultation, $45,000 for DOS, and $310,000 for an array of 16bit language interpreters and compilers. There was no ongoing obligation and no per copy royalties. Microsoft retained the rights to own DOS and its languages and license them to anyone else at any price on any terms. Once application developers started targeting the operating system distributed through IBM's PC, that operating system became the one every other OEM and computer maker also wanted and needed, and they had to come to Microsoft to buy it.

Try this

When negotiating a deal with a larger partner, accept a fixed fee but explicitly retain rights to license your work elsewhere.