I had the transmission in my car fixed, but it is leaking. I took it back and they told me it was the oil leaking. I don't think it is oil that is leaking out.
Remember that engine oil is brown and transmission fluid is reddish with a strong odor. If you are positive that the transmission is leaking, ie. the fluid level is dropping, you need to fix the leak at once. You may just have residual fluid left on the vehicle after the initial repair. They should have thoroughly cleaned it, but they may have not. I recommend letting another technician, such as one from YourMechanic inspect the leak issue and have this problem repaired.