I have to agree with the others on this one...By going ahead and accepting the breeding you were accepting the new terms...If you were unhappy with what was going on you really should have just forfeited your breeding and walked away...
As a lawyer I am confused why you didn't draw up a written contract with the owner of the stud dog...
...Sounds like to me you are looking for a loophole in hopes of not paying the agreed upon price...
Bottom line is you didn't walk away...You got your bitch bred...and you owe the stud fee!