«لِفُلَانٍ عَلَيَّ أَلْفٌ»
“I owe a thousand to so-and-so.”
Ala indicates liability: the amount is acknowledged as a debt borne by the speaker.
By contrast, phrases such as “with me” or “in my possession” may report custody or possession without the same admission of indebtedness.
A legal analysis must distinguish a statement about where money is held from an acknowledgement that the holder must repay it as a debt.