Birmingham allow Hart to go back to City for 3 games, City repay the loan fee.
City are chasing 4th, Birmingham are chasing.....nothing.
At the end of the season Birmingham should have first choice over signing Hart on a reduced fee or on loan again as a thank you from City for helping them out whilst Cit shop around for a replacement 'keeper in the summer.
Should Birmingham refuse, they keep Hart for their remaining matches and next season can kiss ANY chance of signing Hart, loan or otherwise, goodbye.
Regardless, Joe Hart WILL return to Manchester City in the summer, so does he want to rot in the reserves, or play a part? If he does well he could dislodge Given, who's out for up to 8 months.
It's just, good business.