I agree with @Mike9241; this scenario presents as a transfer in part a gift and in part a sale (the presumption is that the FMV exceeds the balance of the loan), which is controlled by Treas. Reg. §1.1015-4. This issue would not have arisen had you simply paid the loan back in cash, but you transferred property in exchange for loan being retired, essentially selling a 50% interest in the property.
You need professional guidance here because you may need to recognize gain (including depreciation recapture) on this transfer, your basis and your mother's basis will almost certainly differ, and your reporting for the rental going forward will change. It would also be appropriate to draft a quitclaim deed granting 50% of the property to your mother (most likely as tenants in common, but that's another discussion).
The yield here is that consultations (for you and your mother) with local tax and legal professionals is necessary for various reasons.
