To put free not. And impossible to buy.
I personally faced when my super-program was required to work a certain utility, which only works in DOS (she needs the COM ports in real time, you see). And DOS license to purchase is not free to supply.
With FreeDOS, the situation is even more complicated because it is unclear who owns the rights to FreeDOS, and if people come in masks, they have nothing to prove, followed by a command: "Remove hands from the keyboard" and for refusing — butt on the forehead.
I had to rewrite the code and to drive the tool straight into the BOOT-sector of floppy without DOS. It turned out to be easier than finding/buying a DOS-compatible OSes.