He said: "The money we'll receive isn't linked to appearances or any other kind of add-ons. We're getting half the transfer fee now and the balance within 12 months.
There's an additional payment if Steven is sold on and we'll get a good percentage of any fee if he does move away.
"He refused the offer of a new contract we made to him last week and at the same time told us he wasn't interested in moving to England for family reasons and didn't want to go to any other Scottish club.
Under those circumstances we decided it was better to accept Rangers' offer than hold an unhappy player. There was also the fear of FIFA's Article 17 being imposed on us by the player when his contract began its last year in March 2009."